Add arch-x86_64/bionic.

This is basically half of I5de76f6c46ac87779f207d568a86bb453e2414de from
Pavel Chupin <pavel.v.chupin@intel.com>, but with the stock upstream
setjump/sigsetjmp and H.J. Lu's suggested changes to __rt_sigreturn.

Change-Id: I8167ec228faeb2065391e5bec0413cca662f3d33