gdb/testsuite: allow_hipcc_tests tests the hipcc compiler

gdb/testsuite: allow_hipcc_tests tests the hipcc compiler

Update allow_hipcc_tests so all gdb.rocm tests are skipped if we do not
have a working hipcc compiler available.

To achieve this, adjust gdb_simple_compile to ensure that the hip
program is saved in a ".cpp" file before calling hipcc otherwise
compilation will fail.

One thing to note is that it is possible to have a hipcc installed with
a CUDA backend.  Compiling with this back-end will successfully result
in an application, but GDB cannot debug it (at least for the offload
part). In the context of the gdb.rocm tests, we want to detect such
situation where gdb_simple_compile would give a false positive.

To achieve this, this patch checks that there is at least one AMDGPU
device available and that hipcc can compile for this or those targets.
Detecting the device is done using the rocm_agent_enumerator tool which
is installed with the all ROCm installations (it is used by hipcc to
detect identify targets if this is not specified on the comand line).

This patch also makes the allow_hipcc_tests proc a cached proc.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Approved-By: Simon Marchi <simon.marchi@efficios.com>