[libc] Re-enable functions from signal.h and re-enable abort.

They were disabled because we were including linux/signal.h from our
signal.h. Linux's signal.h is not designed to be included from user
programs as it causes a lot of non-standard name pollution. Also, it is
not self-contained. This change defines types and macros relevant for
signal related syscalls within libc's headers and removes inclusion of
Linux headers.

This patch enables the funtions only for x86_64. They will be enabled
for aarch64 also in a follow up patch after testing.

Reviewed By: abrachet, lntue

Differential Revision: https://reviews.llvm.org/D134567