Skip setting HW watchpoint if skip_hw_watchpoint_multi_tests in gdb.base/break-idempo...

Skip setting HW watchpoint if skip_hw_watchpoint_multi_tests in gdb.base/break-idempotent.exp

Hi,
I see this fails below on arm linux native testing and remote testing
with "set remote hardware-watchpoint-limit 1",

 rwatch global^M
 There are not enough available hardware resources for this watchpoint.^M
 (gdb) FAIL: gdb.base/break-idempotent.exp: always-inserted off: rwatch: twice: rwatch global

gdb.base/break-idempotent.exp sets two breakpoints/watchpoints on the
same address.  GDB isn't smart enough calculate these two HW
watchpoints can fit in one HW debug register, so the error message
above isn't necessary (there is one HW watchpoint register on arm).
Because target_ops interface can_use_hardware_watchpoint doesn't
pass enough information to the target backend.

Note that if I don't "set remote hardware-watchpoint-limit 1" in
remote testing, this test passes without fails.  However without
"set remote hardware-watchpoint-limit 1", many other watchpoint
tests fail.

This patch is to add a check to skip_hw_watchpoint_multi_tests
for rwatch and awatch.  We can add such check for watch as well,
but GDB is able to switch to software watchpoint if HW resource
isn't available, it doesn't cause any fail, I decide not to skip.

gdb/testsuite:

2015-04-30  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/break-idempotent.exp: If
	skip_hw_watchpoint_multi_tests returns true, skip the tests
	on "rwatch" and "awatch".