tsan: update Go x86 build rules to back off to sse3

[thanm]

Summary

This is a partial revert of https://reviews.llvm.org/D106948, changing
just the Go build rules to remove -msse4.2 and revert back to -msse3,
so as to preserve support for older x86 machines. More details at
https://github.com/golang/go/issues/53743.

[dvyukov]

compiler-rt/lib/tsan/go/buildgo.sh

62–64

Please add some comment with references here as to why we use sse3.

[dvyukov]

Otherwise looks good.

[thanm]

compiler-rt/lib/tsan/go/buildgo.sh

62–64

Done.

[thanm]

Added comments.

[thanm]

Correct indentation (spaces -> tabs).

compiler-rt/lib/tsan/go/build.bat

gcc ^
   -DGetProcessMemoryInfo=K32GetProcessMemoryInfo ^
   -Wno-error=attributes ^
   -Wno-attributes ^
   -Wno-format ^
   -Wno-maybe-uninitialized ^
   -DSANITIZER_DEBUG=0 ^
   -O3 ^
   -fomit-frame-pointer ^
-  -msse4.2 ^
+  -msse3 ^
   -std=c++14
+
+rem "-msse3" used above to ensure continued support of older
+rem cpus (for now), see https://github.com/golang/go/issues/53743.

compiler-rt/lib/tsan/go/buildgo.sh

SRCS="
     ../../sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp
     ../../sanitizer_common/sanitizer_stoptheworld_netbsd_libcdep.cpp
     "
   if [ "`uname -a | grep ppc64le`" != "" ]; then
     SUFFIX="linux_ppc64le"
     ARCHCFLAGS="-m64 -mcpu=power8 -fno-function-sections"
   elif [ "`uname -a | grep x86_64`" != "" ]; then
     SUFFIX="linux_amd64"
-    ARCHCFLAGS="-m64 -msse4.2"
+    # -msse3 used below to ensure continued support of older
+                # cpus for now, see https://github.com/golang/go/issues/53743.
+    ARCHCFLAGS="-m64 -msse3"
     OSCFLAGS="$OSCFLAGS -ffreestanding -Wno-unused-const-variable -Wno-unknown-warning-option"
   elif [ "`uname -a | grep aarch64`" != "" ]; then
     SUFFIX="linux_arm64"
     ARCHCFLAGS=""
   elif [ "`uname -a | grep -i mips64`" != "" ]; then
     if [ "`lscpu | grep -i Little`" != "" ]; then
       SUFFIX="linux_mips64le"
       ARCHCFLAGS="-mips64 -EL"