MIPS32: Fix MipsInstructionSetFeatures::FromVariant()

Make it possible to completely override build-time instruction
set features with the provided variant string. Add sanity checks.
Comment on the "default" variant behavior.

Tested:
- test-art-host-gtest
- test-art-target-gtest and test-art-target-run-test-optimizing on:
  - MIPS32R6 (2nd arch) QEMU
  - CI20 board

Change-Id: I2470b7115a5b5e333e2e7a156e68c39945fb02e9