misc/cgo/testcarchive: don't use == for string equality in C code

Backport of https://golang.org/cl/300993.

For PR go/99553

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/301458

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-bf35249a7c752836741b1cab43a312f87916fcb0
+2f281eb24ef256a2d3bb9fc1a7ef964d82b40182
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/misc/cgo/testcarchive/testdata/main_unix.c

@@ -36,7 +36,7 @@ int install_handler() {
 		return 2;
 	}
 	// gccgo does not set SA_ONSTACK for SIGSEGV.
-	if (getenv("GCCGO") == "" && (osa.sa_flags&SA_ONSTACK) == 0) {
+	if (getenv("GCCGO") == NULL && (osa.sa_flags&SA_ONSTACK) == 0) {
 		fprintf(stderr, "Go runtime did not install signal handler\n");
 		return 2;
 	}