Build the linker with -D_USING_LIBCXX

This macro is necessary for using stdatomic.h and <atomic> in the same
libc++-based program. <atomic> is used implicitly by other STL headers,
and stdatomic.h is used (or will be used) in headers shared with libc.
libc only has access to stdatomic.h because it can't use an STL.

Bug: http://b/78026329
Test: bionic unit tests
Change-Id: I89b128df08b478287ef4e2867319e74dbcc30d4e