Use CXXCOMPILE in gold/testsuite/Makefile for c++ testcases

Use CXXCOMPILE in gold/testsuite/Makefile for c++ testcases

I was playing with passing -std=c99 to an older version of gcc by
using CC="gcc-4 -std=c99", and ran into
cc1plus: error: command line option ‘-std=c99’ is valid for C/ObjC but
not for C++ [-Werror]
This obvious fix uses the correct compiler for a number of gold
testcases.

	* testsuite/Makefile.am (export_dynamic_plugin.o): Use CXXCOMPILE.
	(plugin_test_wrap_symbols_1.o): Likewise.
	(plugin_test_wrap_symbols_2.o): Likewise.
	* testsuite/Makefile.in: Regenerate.