Remove architecture specific sigreturn.S files.

[brooks]

Summary

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.

Test Plan

universe build

[kib]

Can you show me the output of
readelf -l /lib/libc.so.7
for the patched library ?

[brooks]

• Work correctly in a clean tree.

[brooks]

In D14953#314405, @kib wrote:

Can you show me the output of
readelf -l /lib/libc.so.7
for the patched library ?

See paste P162