[BOLT][RISCV] Set minimum function alignment to 2 for RVC (#69837)

In #67707, the minimum function alignment on RISC-V was set to 4. When
RVC (compressed instructions) is enabled, the minimum alignment can be
reduced to 2.

This patch implements this by delegating the choice of minimum alignment
to a new `MCPlusBuilder::getMinFunctionAlignment` function. This way,
the target-dependent code in `BinaryFunction` is minimized.