htl: Fix linking static tests by factorizing the symbols list

libpthread_syms.a will contain the symbols that libc tries to get from
libpthread, to be used by the system, but also by tests.

* htl/libpthread.a, htl/libpthread_pic.a: Link libpthread_syms.a and Move EXTERN
references to...
* htl/libpthread_syms.a: ... new file. Add missing
__pthread_enable_asynccancel reference.
* htl/Makefile: Install libpthread_syms.a and link it into static tests.