RISC-V: Improve li expansion for better code density.

[Kito Cheng]

ChangeLog:
v2:
  - Use vasprintf in md_assemblef.
  - Fix typo.
  - Fix wrong logic for RV64.
  - Added a testcase on GCC side
    - https://gcc.gnu.org/ml/gcc-patches/2019-08/msg01614.html