[gdb/testsuite] Add can_compile rust

[gdb/testsuite] Add can_compile rust

If I deinstall the rust compiler, I get:
...
gdb compile failed, default_target_compile: Can't find rustc --color never.
UNTESTED: gdb.rust/watch.exp: failed to prepare
...

Fix this by adding can_compile rust, and using it in allow_rust_tests, such
that we have instead:
...
UNSUPPORTED: gdb.rust/watch.exp: require failed: allow_rust_tests
...

Since the rest of the code in allow_rust_tests is also about availability of
the rust compiler, move it to can_compile.

Tested on x86_64-linux.