RISC-V: enable have_nonsteppable_watchpoint by default (#160)

* RISC-V: enable have_nonsteppable_watchpoint by default

The RISC-V debug spec 0.13 recommends that write triggers fire before
the write is committed. If the target follows this behaviour, then
have_nonsteppable_watchpoint needs to be set to 1 so that GDB will step
over the watchpoint before checking if the value has changed.

This patch adds a setshow for have_nonsteppable_watchpoint which
defaults to 1 to match the recommended behaviour. If a target does not
follow this timing, then 'set riscv have_nonsteppable_watchpoint 0'
will need to be issued on the command line.

gdb/ChangeLog:

	* riscv-tdep.c (set_have_nonsteppable_watchpoint): add
	callback for 'set riscv have_nonsteppable_watchpoint'
	(riscv_gdbarch_init): initialise gdbarch setting for
	have_nonesteppable_watchpoint

* Add changes following comments on upstream patch

Adds a show method for 'show riscv have-nonsteppable-watchpoint'
plus additional comment and formatting fixes.

gdb/ChangeLog:

	* riscv-tdep.c (show_have_nonsteppable_watchpoint): Add
	callback for 'show riscv have-nonsteppable-watchpoint'.