gdb/riscv: Provide non-DWARF stack unwinder

gdb/riscv: Provide non-DWARF stack unwinder

Collects information during the prologue scan and uses this to unwind
registers when no DWARF information is available.

This patch has been tested by disabling the DWARF stack unwinders, and
running the complete GDB testsuite against a range of RISC-V targets.
The results are comparable to running with the DWARF unwinders in
place.

gdb/ChangeLog:

	* riscv-tdep.c: Add 'prologue-value.h' include.
	(struct riscv_unwind_cache): New struct.
	(riscv_debug_unwinder): New global.
	(riscv_scan_prologue): Update arguments, capture register details
	from prologue scan.
	(riscv_skip_prologue): Reformat arguments line, move end of
	prologue calculation into riscv_scan_prologue.
	(riscv_frame_cache): Update return type, create
	riscv_unwind_cache, scan the prologue, and fill in remaining cache
	details.
	(riscv_frame_this_id): Use frame id computed in riscv_frame_cache.
	(riscv_frame_prev_register): Use the trad_frame within the
	riscv_unwind_cache.
	(_initialize_riscv_tdep): Add 'set/show debug riscv unwinder'
	flag.