gdb/testsuite: look for hipcc in env(ROCM_PATH)

If the hipcc compiler cannot be found in dejagnu's tool_root_dir, look
for it in $::env(ROCM_PATH) (if set).  If hipcc is still not found,
fallback to "hipcc" so the compiler will be searched in the PATH.  This
removes the fallback to the hard-coded "/opt/rocm/bin" prefix.

This change is done so ROCM tools are searched in a uniform manner.

Approved-By: Simon Marchi <simon.marchi@efficios.com>