[gdb/testsuite] Unsupport gdb.rust for remote host

[gdb/testsuite] Unsupport gdb.rust for remote host

With test-case gdb.rust/watch.exp and remote host I run into:
...
Executing on host: gcc   watch.rs  -g  -lm   -o watch    (timeout = 300)
  ...
ld:watch.rs: file format not recognized; treating as linker script
ld:watch.rs:1: syntax error
  ...
UNTESTED: gdb.rust/watch.exp: failed to prepare
...

The problem is that find_rustc returns "" for remote host, so we fall back to gcc, which fails.

Fix this by returning 0 in allow_rust_tests for remote host.

Tested on x86_64-linux.