target/mips: Optimize regnames[] arrays

target/mips: Optimize regnames[] arrays

Since all entries are no more than 3/4/6 bytes (including nul
terminator), can save space and pie runtime relocations by
declaring regnames[] as array of 3/4/6 const char.

Inspired-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210617174323.2900831-10-f4bug@amsat.org>