[sanitizers] Update sanitizers test to better match glibc internals

One of the tests relying on sem_t's layout gets the wrong value for versions of
glibc newer than 2.21 on platforms that don't have 64-bit atomics (e.g. ARM).

This commit fixes the test to work with:
* versions of glibc >= 2.21 on platforms with 64-bit atomics: unchanged
* versions of glibc >= 2.21 on platforms without 64-bit atomics: the semaphore
value is shifted by SEM_VALUE_SHIFT (which is set to 1 in glibc's internal
headers)
* versions of glibc < 2.21: unchanged

See the glibc 2.23 sources:
* sysdeps/nptl/internaltypes.h (struct new_sem for glibc >= 2.21 and
                                struct old_sem for glibc < 2.21)
* nptl/sem_getvalue.c

This was uncovered on one of the new buildbots that we are trying to move to
production.

Differential Revision: https://reviews.llvm.org/D24766

llvm-svn: 282061