Enable gdb.cp/ambiguous.exp with GCC and clang

Enable gdb.cp/ambiguous.exp with GCC and clang

gdb.cp/ambiguous.exp failed to build using clang with the following
error:

 gdb compile failed, /gdbtest/src/gdb/testsuite/gdb.cp/ambiguous.cc:70:36:
   warning: direct base 'A1' is inaccessible due to ambiguity:
     class JVA1 -> class KV -> class A1
     class JVA1 -> class A1 [-Winaccessible-base]
 class JVA1 : public KV, public LV, public A1 {
                                   ^~~~~~~~~

This commit builds this testcase with -Wno-inaccessible-base when
using clang, to avoid this failure.

Furthermore, gdb.cp/ambiguous.exp has been disabled when using GCC
since 1998.  This commit enables this testcase, building with
-Wno-inaccessible-base when using GCC >= 10.1, and -w otherwise.

gdb/testsuite/ChangeLog:

	* gdb.cp/ambiguous.exp: Enable test when compiling with GCC.
	Add additional_flags=-Wno-inaccessible-base when compiling
	with GCC >= 10.1 or clang.  Add additional_flags=-w when
	compiling with GCC < 10.