syscall: mksysnum_linux.pl: run syscall numbers through GCC

This will skip system call numbers that are ifdef'ed out in unistd.h,
as occurs on PPC.

Change-Id: I88e640e4621c7a8cc266433f34a7b4be71543ec9
Reviewed-on: https://go-review.googlesource.com/9966Reviewed-by: Minux Ma <minux@golang.org>

src/syscall/mkall.sh

@@ -179,7 +179,7 @@ linux_amd64)
 linux_arm)
 	mkerrors="$mkerrors"
 	mksyscall="./mksyscall.pl -l32 -arm"
-	mksysnum="curl -s 'http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/arch/arm/include/uapi/asm/unistd.h' | ./mksysnum_linux.pl"
+	mksysnum="curl -s 'http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/arch/arm/include/uapi/asm/unistd.h' | ./mksysnum_linux.pl -"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 linux_arm64)

src/syscall/mksysnum_linux.pl

@@ -28,7 +28,8 @@ sub fmt {
 }
 
 my $prev;
-while(<>){
+open(GCC, "gcc -E -dD $ARGV[0] |") || die "can't run gcc";
+while(<GCC>){
 	if(/^#define __NR_syscalls\s+/) {
 		# ignore redefinitions of __NR_syscalls
 	}