runtime/testdata: fix for C23 nullptr keyword

Backport https://go.dev/cl/620955 from main repo.  Original description:

    src/runtime/testdata/testprogcgo/threadprof.go contains C code with a
    variable called nullptr.  This conflicts with the nullptr keyword in
    the C23 revision of the C standard (showing up as gccgo test build
    failures when updating GCC to use C23 by default when building C
    code).

    Rename that variable to nullpointer to avoid the clash with the
    keyword (any other name that's not a keyword would work just as well).

Change-Id: I2e49fc87be16518215825f564bf5317eff4d4f3f
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/621059
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

libgo/go/runtime/testdata/testprogcgo/threadprof.go

@@ -36,10 +36,10 @@ __attribute__((constructor)) void issue9456() {
 	}
 }
 
-void **nullptr;
+void **nullpointer;
 
 void *crash(void *p) {
-	*nullptr = p;
+	*nullpointer = p;
 	return 0;
 }