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).

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/621059