[compiler-rt] Don't force ASAN_HAS_EXCEPTIONS to be true for all builds

[fjricci]

Summary

This value is already defaulted to true in asan_internal.h.
Allow the value to be overriden in cases where exceptions are unavailable.

[compnerd]

I assume that this has no real effect because ASAN_HAS_EXCEPTIONS defaults to 1 if undefined?

[fjricci]

As far as I can tell, that's correct. From asan_internal.h:

// If set, asan will intercept C++ exception api call(s).
#ifndef ASAN_HAS_EXCEPTIONS
# define ASAN_HAS_EXCEPTIONS 1
#endif

compiler-rt/trunk/lib/asan/CMakeLists.txt

 set(ASAN_PREINIT_SOURCES
   asan_preinit.cc)
 
 include_directories(..)
 
 set(ASAN_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 append_rtti_flag(OFF ASAN_CFLAGS)
 
-set(ASAN_COMMON_DEFINITIONS
-  ASAN_HAS_EXCEPTIONS=1)
-
 set(ASAN_DYNAMIC_LINK_FLAGS)
 
 if(ANDROID)
   list(APPEND ASAN_COMMON_DEFINITIONS
     ASAN_LOW_MEMORY=1)
 # On Android, -z global does not do what it is documented to do.
 # On Android, -z global moves the library ahead in the lookup order,
 # placing it right after the LD_PRELOADs. This is used to compensate for the fact