Remove duplicate copies of trivial getcontextx.c

Only i386 and amd64 provide a non-trivial __getcontextx(). Use a common
trivial implementation in gen/ for other architectures, rather than
copying the file to each MD subdirectory.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1472