gdb: LoongArch: Clean up loongarch_iterate_over_regset_sections()

Define a new variable gpsize as gprsize * LOONGARCH_LINUX_NUM_GREGSET
to replace the related code in the first cb(), and also make use of
tabs and spaces in indentation to force the proper alignment of code,
then remove the empty line at the end of the function.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>