LoongArch: Use functions instead of magic numbers.

  Replace the magic numbers in gas(tc-loongarch.c) and
  bfd(elfnn-loongarch.c) with the functions defined in
  the howto table(elfxx-loongarch.c).

  gas/
    * config/tc-loongarch.c: use functions.

  bfd/
    * elfnn-loongarch.c: use functions.
    * elfxx-loongarch.c: define functions.
    * elfxx-loongarch.h