Weak doesn't work the same on PE/COFF as on ELF, they are only weak
references. Specifically, since nothing else pulls in the object which
contains pthread_mutexattr_init() (and coming from the C library, that is
the only thing that object contains), means that it ends up as 0
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>