libsys: expose __libc_sig{action,protmask,suspend}

[brooks]

Summary

These are weak symbols provided for libc's internal use. It may be
that we simply want to call sig<foo> directly instead, but for now keep
the weak symbols in libsys.

To enable this, alter i386's (sig)setjmp to allow external linkage.

[kib]

lib/libc/i386/gen/setjmp.S

52

Then why e.g. amd64 does not require similar handling?

[brooks]

I'm going to pull the implementations of these back into libc so this won't be needed.