[tilegx] use "sra" instead of "srl", although currently those "key" will not be negative.
[tilegx] check reg collision in MacroAssembler::CopyFields.
[tilegx] Implemented SoftFloat TBD in StubCompiler::GenerateStoreTransition.
[tilegx] Remove "UNIMPLEMENTED" marker in MathFloor.

  When returned null, libm will be invoked.