[RISCV] RISCVELFTargetObjectFile: use 2-byte alignment for .text if RVC

For the "C" Standard Extension/Zca, D45560 enabled 2-byte alignment for
assembly output (e.g. `clang -S a.c`) and D102052 enabled 2-byte alignment for
assembly input and object file output (e.g. `clang -c a.s`).

This patch ports the behavior for code generation and object file output by
adding RISCVELFTargetObjectFile::getTextSectionAlignment (e.g. `clang -c a.c`).

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D150240