linux(4): Fix stack unwinding across signal frame on x86_64

Get rid of using register numbers which is undefined in libunwind
on x86_64.

Differential Revision: https://reviews.freebsd.org/D40156
MFC after: 1 month

(cherry picked from commit 78c2e58fa5b0aa4518d79d8155c0513b9a04f531)