[libc] Use Linux errno and signal strings for Fuchsia

The exact set of supported values is determined by the <errno.h>
and <signal.h> headers, which don't (yet) come from llvm-libc on
Fuchsia. The mappings of SIG* and E* codes to psignal/strsignal
and perror/strerror text used in Fuchsia libc today is the same
as for Linux.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D150026