Detect the magic address of EXC_RETURN in ARM coretx-m profile

Detect the magic address of EXC_RETURN in ARM coretx-m profile

On ARMv6-M and ARMv7-M, the exception return address is sort of magic
address defined by the manual.  This patch is to let GDB well handle
these magic addresses.

2016-09-27  Fredrik Hederstierna  <fredrik.hederstierna@verisure.com>

	* arm-tdep.c (arm_m_addr_is_magic): New function.
	(arm_addr_bits_remove): Call arm_m_addr_is_magic.
	(arm_m_exception_unwind_sniffer): Likewise.