RISC-V: Linux signal frame support.

RISC-V: Linux signal frame support.

Add support for recognizing signal trampolines, parsing the signal frame,
and reading register values from it.

	gdb/
	* riscv-linux-tdep.c: Include tramp-frame.h and trad-frame.h.
	(riscv_linux_sigframe_init): Declare.
	(RISCV_INST_LI_A7_SIGRETURN, RISCV_INT_ECALL): New.
	(riscv_linux_sigframe): New.
	(SIGFRAME_SIGINFO_SIZE, UCONTEXT_MCONTEXT_OFFSET): New.
	(riscv_linux_sigframe_init): Define.
	(riscv_linux_init_abi): Call tramp_frame_prepend_unwinder.