os-wrappers-test: Make syscall intercepts work with sanitizers

Sanitizers need to intercept syscalls in the compiler run-time library, as
do these tests. We try to make this work by using dlsym(RTLD_NEXT) to find
the next definition in the chain, but here this approach won't work because
the compiler run-time library is linked into the same elf object as the test
interceptors are.

The sanitizer library supports this by giving the intercept functions a
prefix and making them only weakly alias the real names, so our interceptors
can call the sanitizers interceptors explicitly, which will then use dlsym
to call the real function.

By making our declarations of the sanitizer interceptor function weak we can
handle any combination of intercepts (including none, if there is no
sanitizer). If our declaration is resolves to a NULL pointer, we just use
dlsym.

Signed-off-by: Fergus Dall <sidereal@google.com>