Don't skip hw breakpoint/watchpoint tests for aarch64 target

Don't skip hw breakpoint/watchpoint tests for aarch64 target

This patch is to let skip_hw_breakpoint_tests and skip_hw_watchpoint_tests
return 0 for aarch64 target, since aarch64 has HW watchpoint and
breakpoint registers.

With this patch applied, about 1560 watchpoint/breakpoint related tests
become enabled on aarch64-linux native testing.

gdb/testsuite:

2015-06-22  Yao Qi  <yao.qi@linaro.org>

	* lib/gdb.exp (skip_hw_breakpoint_tests): Return 0 for target
	aarch64*-*-*.
	(skip_hw_watchpoint_tests): Likewise.