tsan: enable -msse4 when compiling tests

[dvyukov]

Summary

Vector SSE accesses make compiler emit __tsan_[unaligned_]read/write16 callbacks.
Make it possible to test these.

[melver]

Is COMPILER_RT_HAS_MSSE4_2_FLAG only true if the host has support?

If not, I'm sure there are people who run something that doesn't support SSE 4.2. For us it's fine, so let's see which bots complain.

[dvyukov]

In D114591#3153893, @melver wrote:

Is COMPILER_RT_HAS_MSSE4_2_FLAG only true if the host has support?

If not, I'm sure there are people who run something that doesn't support SSE 4.2. For us it's fine, so let's see which bots complain.

We use COMPILER_RT_HAS_MSSE4_2_FLAG to add -msee4 when building the runtime itself. So I assume it works one way or another.

compiler-rt/test/tsan/CMakeLists.txt

foreach(arch ${TSAN_TEST_ARCH})
 
   set(TSAN_TEST_TARGET_ARCH ${arch})
   string(TOLOWER "-${arch}" TSAN_TEST_CONFIG_SUFFIX)
   get_test_cc_for_arch(${arch} TSAN_TEST_TARGET_CC TSAN_TEST_TARGET_CFLAGS)
 
   string(REPLACE ";" " " LIBDISPATCH_CFLAGS_STRING " ${COMPILER_RT_TEST_LIBDISPATCH_CFLAGS}")
   string(APPEND TSAN_TEST_TARGET_CFLAGS ${LIBDISPATCH_CFLAGS_STRING})
 
+  if (COMPILER_RT_HAS_MSSE4_2_FLAG)
+    string(APPEND TSAN_TEST_TARGET_CFLAGS " -msse4.2 ")
+  endif()
+
   string(TOUPPER ${arch} ARCH_UPPER_CASE)
   set(CONFIG_NAME ${ARCH_UPPER_CASE}Config)
 
   configure_lit_site_cfg(
     ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.py.in
     ${CMAKE_CURRENT_BINARY_DIR}/${CONFIG_NAME}/lit.site.cfg.py
     MAIN_CONFIG
     ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py