Fix gdb.base/list-ambiguous.exp with Clang

The ambiguous variable parts of gdb.base/list-ambiguous.exp failed
when built with Clang because the variable in question was unused
and was optimized out.  This commit adds __attribute__ ((used)) to
both definitions.

gdb/testsuite/ChangeLog:

	* gdb.base/list-ambiguous0.c (ambiguous_var): Add
	__attribute__ ((used)).
	* gdb.base/list-ambiguous1.c (ambiguous_var): Likewise.