RISC-V: Don't allow unaligned breakpoints.

RISC-V: Don't allow unaligned breakpoints.

Some hardware doesn't support unaligned accesses, and a bare metal target
may not have an unaligned access trap handler.  So if the PC is 2-byte
aligned, then use a 2-byte breakpoint to avoid unaligned accesses.

Tested on native RV64GC Linux with gdb testsuite and cross on spike
simulator and openocd with riscv-tests/debug.

	gdb/
	* riscv-tdep.c (riscv_breakpoint_kind_from_pc): New local unaligned_p.
	Set if pcptr if unaligned.  Return 2 if unaligned_p true.  Update
	debugging messages.