x86: Use sysdep.o from libc.a in static libraries

Static libraries can use the sysdep.o copy in libc.a without
a performance penalty.  This results in a visible difference
if libpthread.a is relinked into a single object file (which
is needed to support libraries which check for the presence
of certain symbols to enable threading support, which generally
fails with static linking unless libpthread.a is relinked).

(cherry picked from commit e67330ab)