syscall: Fix generator for Linux syscalls.

In golang.org/cl/14449 the `getdents' system call got changed to use
_SYS_getdents as a layer of indirection instead of SYS_GETDENTS64 for
compatibility with mips64, but this broke mksyscall.pl, which then
died with with:
  syscall_linux.go:840: malformed //sys declaration

Change-Id: Icb61965d8730f6e81f9fb0fa28c7bab635470f09
Reviewed-on: https://go-review.googlesource.com/20051Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/mksyscall.pl

@@ -100,7 +100,7 @@ while(<>) {
 	# Line must be of the form
 	#	func Open(path string, mode int, perm int) (fd int, errno error)
 	# Split into name, in params, out params.
-	if(!/^\/\/sys(nb)? (\w+)\(([^()]*)\)\s*(?:\(([^()]+)\))?\s*(?:=\s*((?i)SYS_[A-Z0-9_]+))?$/) {
+	if(!/^\/\/sys(nb)? (\w+)\(([^()]*)\)\s*(?:\(([^()]+)\))?\s*(?:=\s*((?i)_?SYS_[A-Z0-9_]+))?$/) {
 		print STDERR "$ARGV:$.: malformed //sys declaration\n";
 		$errors = 1;
 		next;