[gdb/testsuite] Handle missing gdc in gdb.dlang/dlang-start.exp

[gdb/testsuite] Handle missing gdc in gdb.dlang/dlang-start.exp

On openSUSE Leap 15.4, I get:
...
Running gdb.dlang/dlang-start.exp ...
gdb compile failed, default_target_compile: Can't find gdc.
UNTESTED: gdb.dlang/dlang-start.exp: failed to prepare
...

Fix this by:
- introducing a new proc can_compile, and
- requiring "can_compile d" in the test-case,
such that I have instead:
...
Running gdb.dlang/dlang-start.exp ...
UNSUPPORTED: gdb.dlang/dlang-start.exp: require failed: can_compile d
...

Tested on x86_64-linux, on openSUSE Leap 15.4 and Fedora 37.