[compiler-rt][cmake] Build unit tests conditionally with _FILE_OFFSET_BITS=64 and _LARGEFILE_SOURCE

[sdardis]

Summary

The sanitizer library unit tests for libc can get a different definition
of 'struct stat' to what the sanitizer library is built with for certain
targets.

For MIPS the size element of 'struct stat' is after a macro guarded
explicit padding element.

This patch resolves any possible inconsistency by adding the same
_FILE_OFFSET_BITS=64 and _LARGE_SOURCE with the same
conditions as the sanitizer library to the build flags for the unit tests.

This resolves a recurring build failure on the MIPS buildbots due to
'struct stat' defintion differences.

[slthakur]

LGTM

compiler-rt/trunk/lib/sanitizer_common/tests/CMakeLists.txt

 
 # Sanitizer_common unit tests testsuite.
 add_custom_target(SanitizerUnitTests)
 set_target_properties(SanitizerUnitTests PROPERTIES FOLDER "Compiler-RT Tests")
 
 # Adds sanitizer tests for architecture.
 macro(add_sanitizer_tests_for_arch arch)
   get_target_flags_for_arch(${arch} TARGET_FLAGS)
+
+  # If the sanitizer library was built with _FILE_OFFSET_BITS=64 we need
+  # to ensure that the library and tests agree on the layout of certain
+  # structures such as 'struct stat'.
+  if( CMAKE_SIZEOF_VOID_P EQUAL 4 )
+    list(APPEND TARGET_FLAGS "-D_LARGEFILE_SOURCE")
+    list(APPEND TARGET_FLAGS "-D_FILE_OFFSET_BITS=64")
+  endif()
+
   set(SANITIZER_TEST_SOURCES ${SANITIZER_UNITTESTS}
                              ${COMPILER_RT_GTEST_SOURCE})
   set(SANITIZER_TEST_COMPILE_DEPS ${SANITIZER_TEST_HEADERS})
   if(NOT COMPILER_RT_STANDALONE_BUILD)
     list(APPEND SANITIZER_TEST_COMPILE_DEPS gtest)
   endif()
   set(SANITIZER_TEST_OBJECTS)
   foreach(source ${SANITIZER_TEST_SOURCES})