target/mips: Implement emulation of nanoMIPS LLWP/SCWP pair

target/mips: Implement emulation of nanoMIPS LLWP/SCWP pair

Implement support for nanoMIPS LLWP/SCWP instructions. Beside
adding core functionality of these instructions, this patch adds
support for availability control via configuration bit XNP.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Dimitrije Nikolic <dnikolic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Stefan Markovic <smarkovic@wavecomp.com>