cmd/compile: track pragmas in lexer rather than global variables

By using a Pragma bit set (8 bits) rather than 8 booleans, also
reduce Func type size by 8 bytes (208B -> 200B on 64bit platforms,
116B -> 108B on 32bit platforms).

Change-Id: Ibb7e1f8c418a0b5bc6ff813cbdde7bc6f0013b5a
Reviewed-on: https://go-review.googlesource.com/19966
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/compile/internal/gc/racewalk.go

@@ -50,7 +50,7 @@ func ispkgin(pkgs []string) bool {
 }
 
 func instrument(fn *Node) {
-	if ispkgin(omit_pkgs) || fn.Func.Norace {
+	if ispkgin(omit_pkgs) || fn.Func.Pragma&Norace != 0 {
 		return
 	}