[PATCH 0/2] New target hook TARGET_COMPUTE_MULTILIB and implementation for RISC-V

[Kito Cheng]

This patch set allow target to use customized multi-lib mechanism rather than the built-in
multi-lib mechanism.

The motivation of this patch is RISC-V might have very complicated multi-lib re-use
rule*, which is hard to maintain and use current multi-lib scripts,
we even hit the "argument list too long" error when we tried to add more
multi-lib reuse rule.
 
* Here is an example for RISC-V multi-lib rules:
https://gist.github.com/kito-cheng/0289cd42d9a756382e5afeb77b42b73b

V2 Changes:
- NO changes for first patch(TARGET_COMPUTE_MULTILIB part) since first version.
- Handle option other than -march and -mabi for riscv_compute_multilib.