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 <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go