gdb/arm-linux-tdep.c: Handle Thumb2 signal trampolines

Recent versions of glibc have assembled the signal trampoline code
as Thumb2, which causes gdb to misinterpret them and a number of
testsuite tests to fail. Educate gdb about these trampolines and
get the tests running again.

gdb/ChangeLog:

2014-07-28  Will Newton  <will.newton@linaro.org>

	* arm-linux-tdep.c (THUMB2_SET_R7_SIGRETURN1): New define.
	(THUMB2_SET_R7_SIGRETURN2): Likewise.
	(THUMB2_SET_R7_RT_SIGRETURN1): Likewise.
	(THUMB2_SET_R7_RT_SIGRETURN2): Likewise.
	(THUMB2_EABI_SYSCALL): Likewise.
	(thumb2_eabi_linux_sigreturn_tramp_frame): Create new
	struct tramp_frame.
	(thumb2_eabi_linux_rt_sigreturn_tramp_frame): Likewise.
	(arm_linux_init_abi): Add Thumb2 tramp frame unwinders.