Unbreak ptrace on x86_64

Unbreak ptrace on x86_64

The asm code was calling __tls_get_addr in order to set errno to 0
before the system call and that was trashing registers holding the
syscall arguments. Don't bother setting errno to 0 anymore (the
in-tree users are careful to set that themselves when appropriate).
While here, declare it properly as a weak symbol. At this point we
could just drop the hand-written assembly.

Proxies-to-x86_64-dfly: Antonio Huete Jimenez, Jan Lentfer
Dragonfly-bug: http://bugs.dragonflybsd.org/issue1571