LoongArch: add .option directive

In some cases we may want to use different options only for certain
assembly, so the .option directive is added to control the assembler
options.

.option can accept 4 parameters:

push
pop
  Pushes or pops the current option stack. They limit the scope of
  option changes so that they do not affect other parts of the assembly
  file.

relax
norelax
  Enables or disables relaxation.
6 files changed