gdb/riscv: Improve non-dwarf stack unwinding

gdb/riscv: Improve non-dwarf stack unwinding

This commit improves the prologue scanning stack unwinder, to better
support AUIPC, LUI, and more variants of ADD and ADDI.

This allows unwinding over frames containing large local variables,
where the frame size does not fit into a single instruction immediate,
and is first loaded into a temporary register, before being added to
the stack pointer.

A new test is added that tests this behaviour.  As there's nothing
truely RiscV specific about this test I've added it into gdb.base, but
as this depends on target specific code to perform the unwind it is
possible that some targets might fail this new test.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_insn::decode): Decode c.lui.
	(riscv_scan_prologue): Split handling of AUIPC, LUI, ADD, ADDI,
	and NOP.

gdb/testsuite/ChangeLog:

	* gdb.base/large-frame-1.c: New file.
	* gdb.base/large-frame-2.c: New file.
	* gdb.base/large-frame.exp: New file.
	* gdb.base/large-frame.h: New file.