Remove architecture specific sigreturn.S files.

All of these files are identical (modulo license blocks and VCS IDs) to
the files generated by lib/libc/sys/Makefile.inc and serve no purpose.

Reported by: Ali Mashtizadeh <ali@mashtizadeh.com>
Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14953