target/mips: Update gen_flt_ldst()

Update gen_flt_ldst() in order to reuse the functions for nanoMIPS

Signed-off-by: Yongbok Kim <yongbok.kim@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>