Correctly infer ranlib/ar from cross-gcc

The GCC convention is that if the toolchain is named `$target-gnu-gcc`,
then ar will be available as `$target-gnu-gcc-ar`. The code as written
will infer it to be `$target-gnu-ar`, which won't work.

I'm not sure why the code that existed was written the way it was -- I
don't know of any GCC toolchains where the `-gcc` is stripped out in the
name of ar. The file listing on Debian's [GCC 6.x] and [GCC 10.x] all
show the binaries using the `$target-gnu-gcc-ar` format, as does Arch
Linux's [GCC 12.x].

It may seem odd that the code always adds `-gcc` for that match arm, but
this matches what we do for finding `$CC` and `$CXX` where we always
inject the GNU prefix when target != host.

Also note that there isn't a special `ar` for C++, so even when
`self.cpp == true`, we should use `-gcc-ar`.

Our saving grace, and likely the reason bug reports haven't come in
about this, is that we also checks if the resulting binary name is
executable, and if it isn't we fall back to the default AR instead. This
means the bad heuristic is likely often masked by the presence of
another working default AR.

See also alexcrichton/openssl-src-rs#163.

[GCC 6.x]: https://packages.debian.org/stretch/gcc
[GCC 10.x]: https://packages.debian.org/stable/devel/gcc
[GCC 12.x]: https://archlinux.org/packages/core/x86_64/gcc/

Author: Jon Gjengset

src/lib.rs

@@ -2601,7 +2601,7 @@ impl Build {
         } else if self.get_host()? != target {
             match self.prefix_for_target(&target) {
                 Some(p) => {
-                    let target_ar = format!("{}-ar", p);
+                    let target_ar = format!("{}-gcc-ar", p);
                     if Command::new(&target_ar).output().is_ok() {
                         target_ar
                     } else {