compiler-rt: fix build on ARMv6

compiler-rt has accumulated several regressions that prevent it from building
on ARMv6. It is important to note that there are two major versions of ARMv6:
base ARMv6 and ARMv6K. ARMv6K includes several important new instructions,
such as non-word size atomic operations (ldrexd, strexd, etc.) and the yield
instruction. Most ARMv6 CPUs actually implement ARMv6K, including all those used
in Raspberry Pis, but nixpkgs' "raspberryPi" platform targets base ARMv6.

compiler-rt versions 8-14 fail to build on ARMv6 and ARMv6K. compiler-rt 15 (not
yet in nixpkgs) builds on ARMv6K but not ARMv6. This patch fixes versions 9-14
on both ARMv6 variants. The patches don't apply cleanly to version 8, and I
figured it wasn't worth carrying another version of the patches for such an old
version.

A total of five patches are required to get compiler-rt building on ARMv6:
* armv6-mcr-dmb.patch: use `mcr` to provide the equivalent of `dmb` on ARMv6.
  Included in LLVM 15.
* armv6-sync-ops-no-thumb.patch: prevent certain atomic operation functions
  from using Thumb mode. Included in LLVM 15.
* armv6-no-ldrexd-strexd.patch: don't use ldrexd or strexd, which are not
  available in base ARMv6. Submitted upstream by me.
* armv6-scudo-no-yield.patch: use nop instead of yield on ARMv6 in standalone
  scudo. Required by versions >=13, since they enable standalone scudo.
  Submitted upstream by me.
* armv6-scudo-libatomic.patch: link standlone scudo to libatomic on ARMv6 (and
  any other platforms that need it). Not yet submitted because the backport is
  a bit different from the upstream version and I need to test it.

1 files changed, 52 insertions, 0 deletions

diff --git a/pkgs/development/compilers/llvm/common/compiler-rt/armv6-sync-ops-no-thumb.patch b/pkgs/development/compilers/llvm/common/compiler-rt/armv6-sync-ops-no-thumb.patch
new file mode 100644
index 0000000000000..098a155d448c0
--- /dev/null
+++ b/pkgs/development/compilers/llvm/common/compiler-rt/armv6-sync-ops-no-thumb.patch
@@ -0,0 +1,52 @@
+From 5017de8ba4b1fe985169cf54590e858a9019a91f Mon Sep 17 00:00:00 2001
+From: Khem Raj <raj.khem@gmail.com>
+Date: Fri, 11 Mar 2022 16:25:49 -0800
+Subject: [PATCH] [builtins] Do not force thumb mode directive in
+ arm/sync-ops.h
+
+.thumb_func was not switching mode until [1]
+so it did not show up but now that .thumb_func (without argument) is
+switching mode, its causing build failures on armv6 ( rpi0 ) even when
+build is explicitly asking for this file to be built with -marm (ARM
+mode), therefore use DEFINE_COMPILERRT_FUNCTION macro to add function
+header which considers arch and mode from compiler cmdline to decide if
+the function is built using thumb mode or arm mode.
+
+[1] https://reviews.llvm.org/D101975
+
+Note that it also needs https://reviews.llvm.org/D99282
+
+Reviewed By: peter.smith, MaskRay
+
+Differential Revision: https://reviews.llvm.org/D104183
+---
+ compiler-rt/lib/builtins/arm/sync-ops.h | 6 ++----
+ 1 file changed, 2 insertions(+), 4 deletions(-)
+
+diff --git a/lib/builtins/arm/sync-ops.h b/lib/builtins/arm/sync-ops.h
+index 7a26170741ad2..d914f9d3a1093 100644
+--- a/lib/builtins/arm/sync-ops.h
++++ b/lib/builtins/arm/sync-ops.h
+@@ -16,9 +16,8 @@
+ 
+ #define SYNC_OP_4(op)                                                          \
+   .p2align 2;                                                                  \
+-  .thumb;                                                                      \
+   .syntax unified;                                                             \
+-  DEFINE_COMPILERRT_THUMB_FUNCTION(__sync_fetch_and_##op)                      \
++  DEFINE_COMPILERRT_FUNCTION(__sync_fetch_and_##op)                            \
+   DMB;                                                                         \
+   mov r12, r0;                                                                 \
+   LOCAL_LABEL(tryatomic_##op) : ldrex r0, [r12];                               \
+@@ -31,9 +30,8 @@
+ 
+ #define SYNC_OP_8(op)                                                          \
+   .p2align 2;                                                                  \
+-  .thumb;                                                                      \
+   .syntax unified;                                                             \
+-  DEFINE_COMPILERRT_THUMB_FUNCTION(__sync_fetch_and_##op)                      \
++  DEFINE_COMPILERRT_FUNCTION(__sync_fetch_and_##op)                            \
+   push {r4, r5, r6, lr};                                                       \
+   DMB;                                                                         \
+   mov r12, r0;                                                                 \
+