target/mips: Optimize CPU/FPU regnames[] arrays

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

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-6-f4bug@amsat.org>