ARM/FDPIC: Allow backtrace to cross signal handler

ARM/FDPIC: Allow backtrace to cross signal handler

2021-04-14  Mickael Guene  <mickael.guene@st.com>
	Christophe Lyon  <christophe.lyon@st.com>

	* gdb/arm-linux-tdep.c (THUMB2_SET_R7_RT_SIGRETURN): New define.
	(FDPIC_LDR_R12_WITH_FUNCDESC): New define.
	(FDPIC_LDR_R9_WITH_GOT): New define.
	(FDPIC_LDR_PC_WITH_RESTORER): New define.
	(arm_linux_sigreturn_fdpic_init): New.
	(arm_linux_sigreturn_tramp_frame_fdpic): New.
	(arm_linux_init_abi): Handle signal handler for FDPIC.