[sanitizer] Fix sem_init_glibc.cc test on __HAVE_64B_ATOMIC arches.

glibc can use one of 2 layouts for semaphores: architectures that
don't HAVE_64B_ATOMIC use an uint32_t field with semaphore value,
then a private field, then a waiting thread count field - this is
the layout currently assumed by the test. However, HAVE_64B_ATOMIC
arches use a fused uint64_t field that contains the value in low bits
and waiting thread count in high bits, followed by a private field.

This resulted in taking private field from the wrong offset on 64-bit
atomic platforms (the test still passed, but didn't actually test
the private field). On big-endian platforms, this resulted in a fail,
since the first 4 bytes overlay the thread count field, and not
the value field.

Found while porting ASan to s390x.

Patch by Marcin Kościelnicki.

llvm-svn: 265715