[ASan] Fix use of -asan-instrument-assembly in tests

[foad]

Summary

The option -asan-instrument-assembly is declared in the X86 backend.
If I test on PowerPC configured with LLVM_TARGETS_TO_BUILD=All then the
option is tolerated but ignored.
If I test on PowerPC configured with LLVM_TARGETS_TO_BUILD=PowerPC then
the testsuite fails with:
[ 93%] Generating ASAN_INST_TEST_OBJECTS.gtest-all.cc.powerpc64-inline.o
clang (LLVM option parsing): Unknown command line argument '-asan-instrument-assembly'. Try: 'clang (LLVM option parsing) -help'

Fix this inconsistency by only adding the option if that toolchain was
built with the X86 backend included.

[eugenis]

I just discovered I had a fix for this from half a year ago.
Not sure which one is better.
http://reviews.llvm.org/D4718

[foad]

In D6190#4, @eugenis wrote:

I just discovered I had a fix for this from half a year ago.
Not sure which one is better.
http://reviews.llvm.org/D4718

Thanks! D4718 looks cleaner to me. I assume instrumented assembly will be supported on more targets in the future.

[eugenis]

No, D4718 does not work as TargetMachine.cpp does not get linked into llvm-mc. I'm not sure why, and can't spend more time on this right now. If you feel like it, you could try fixing it the right way, or just land this cmake patch.

lib/asan/tests/CMakeLists.txt

list(APPEND ASAN_UNITTEST_COMMON_CFLAGS
   -DASAN_HAS_EXCEPTIONS=1
   -DASAN_UAR=0)
 
 set(ASAN_BLACKLIST_FILE "${CMAKE_CURRENT_SOURCE_DIR}/asan_test.ignore")
 set(ASAN_UNITTEST_INSTRUMENTED_CFLAGS
   ${ASAN_UNITTEST_COMMON_CFLAGS}
   -fsanitize=address
   "-fsanitize-blacklist=${ASAN_BLACKLIST_FILE}"
-  -mllvm -asan-instrument-assembly
 )
+if(CAN_TARGET_x86_64 OR CAN_TARGET_i386)
+  list(APPEND ASAN_UNITTEST_INSTRUMENTED_CFLAGS -mllvm -asan-instrument-assembly)
+endif()
 
 if(NOT MSVC)
   list(APPEND ASAN_UNITTEST_COMMON_LINKFLAGS --driver-mode=g++)
 endif()
 
 # x86_64 FreeBSD 9.2 additionally requires libc++ to build the tests.
 if(CMAKE_SYSTEM MATCHES "FreeBSD-9.2-RELEASE")
   list(APPEND ASAN_UNITTEST_COMMON_LINKFLAGS "-lc++")