gdb/testsuite: disable gdb.cp/call-method-register.exp when not using gcc

The test gdb.cp/call-method-register.exp assumes that the class will be
placed on a register. However, this keyword has been deprecated since
C++11, and Clang, for instance, does not feel the need to follow it.
Since this test is not usable without this working, this commit marks
this test as untested.

Approved-by: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.cp/call-method-register.exp

@@ -26,6 +26,11 @@ if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
     return -1
 }
 
+if {![test_compiler_info gcc-*-* c++]} {
+    untested "test relies on a gcc extension"
+    return
+}
+
 proc test_call_register_class {} {
     global gdb_prompt