Skip hbreak-unmapped.exp if memory at address 0 is readable

hbreak-unmapped.exp assumes that memory at address 0 is unmapped or
unreadable, but on bare metal or uclinux targets, memory at address
0 is readable.  For example, on arm-none-eabi, the vector table base
address is 0x0.

hbreak *0^M
Hardware assisted breakpoint 3 at 0x0: file
/scratch/yqi/arm-none-eabi-lite/obj/cs3-2014.11-999999-arm-none-eabi-i686-pc-linux-gnu/generated/arm-vector.S,
line 25.^M
(gdb) FAIL: gdb.base/hbreak-unmapped.exp: hbreak *0
info break^M
Num     Type           Disp Enb Address    What^M
3       hw breakpoint  keep y   0x00000000
/scratch/yqi/arm-none-eabi-lite/obj/cs3-2014.11-999999-arm-none-eabi-i686-pc-linux-gnu/generated/arm-vector.S:25^M
(gdb) FAIL: gdb.base/hbreak-unmapped.exp: info break shows hw breakpoint
delete $bpnum

This patch is to check whether address 0 is readable via command 'x 0'.
If it is, skip the test.

gdb/testsuite:

2014-06-06  Yao Qi  <yao@codesourcery.com>

	* gdb.base/hbreak-unmapped.exp: Read memory at address 0.  If
	readable, skip the test.