syscall: make mkall.sh accept file list

syscall: make mkall.sh accept file list

Amend the "mkall.sh -syscalls" implementation to
 - prepend ./ before mksyscalls.pl;
 - accept the optional file list argument.

This is a preparation for CL 416115.

Change-Id: Ib4dc2b4aa0d2dd22a256414864e92f2d2fd957a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/423676
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>