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

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

For https://gcc.gnu.org/PR99553

Change-Id: I42727f024af36103435646e58aca013ace6e1581
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/301458
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

diff --git a/libgo/misc/cgo/testcarchive/testdata/main_unix.c b/libgo/misc/cgo/testcarchive/testdata/main_unix.c
index b23ac1c..bd00f9d 100644
--- a/libgo/misc/cgo/testcarchive/testdata/main_unix.c
+++ b/libgo/misc/cgo/testcarchive/testdata/main_unix.c
@@ -36,7 +36,7 @@
 		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;
 	}