[MSP430] Generate EABI-compliant libcalls

Updates the MSP430 target to generate EABI-compatible libcall names.
As a byproduct, adjusts the hardware multiplier options available in
the MSP430 target, adds support for promotion of the ISD::MUL operation
for 8-bit integers, and correctly marks R11 as used by call instructions.

Patch by Andrew Wygle.

Differential Revision: https://reviews.llvm.org/D32676

llvm-svn: 302820