[asan] Set alignment of __asan_global_* globals to sizeof(GlobalStruct)

When using profiling and ASan together (-fprofile-instr-generate -fcoverage-mapping -fsanitize=address), at least on Darwin, the section of globals that ASan emits (__asan_globals) is misaligned and starts at an odd offset. This really doesn't have anything to do with profiling, but it triggers the issue because profiling emits a string section, which can have arbitrary size.  This patch changes the alignment to sizeof(GlobalStruct).

Differential Revision: https://reviews.llvm.org/D28573

llvm-svn: 291715