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>