target/mips: Implement emulation of nanoMIPS EXTW instruction

target/mips: Implement emulation of nanoMIPS EXTW instruction

Implement emulation of nanoMIPS EXTW instruction. EXTW instruction
is similar to the MIPS r6 ALIGN instruction, except that it counts
the other way and in bits instead of bytes. We therefore generalise
gen_align() function into a new gen_align_bits() function (which
counts in bits instead of bytes and optimises when bits = size of
the word), and implement gen_align() and a new gen_ext() based on
that. Since we need to know the word size to check for when the
number of bits == the word size, the opc argument is replaced with
a wordsz argument (either 32 or 64).

Signed-off-by: James Hogan <james.hogan@mips.com>
Signed-off-by: Yongbok Kim <yongbok.kim@mips.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Stefan Markovic <smarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>