[libc] Fix longjmp's fake return in x86_64

This implements the fake return after a successful longjmp(), as
described in the linux man page:

Following  a  successful  longjmp(), execution continues as if setjmp()
had returned for a second time.  This  "fake"  return  can  be  distin-
guished from a true setjmp() call because the "fake" return returns the
value provided in val.  If the programmer mistakenly passes the value 0
in val, the "fake" return will instead return 1.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146981