[MSP430] Align functions on 2-byte boundary instead of 4.

Summary:
There is no benefit in having the 4-byte alignment, and removing this
restriction can save a lot of space for some applications.

Reviewers: asl, awygle

Reviewed By: awygle

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36165

llvm-svn: 313676