[RISCV] Make it explicit that attributes use the MCSubtargetInfo from TargetMachine. NFC

The MCSTI variable is initialized to TM.getMCSubtargetInfo(), but is
re-assigned in every call to runOnMachineFunction. emitAttributes is
called before any call to runOnMachineFunction, but it's not
immediately obvious.

This patch removes the MCSTI variable, and instead queries
TM.getMCSubtargetInfo() at the time emitAttributes is called.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D142037