[libc] Start to refactor riscv platform abstraction to support both 32 and 64 bits versions

This patch enables the compilation of libc for rv32 by unifying the
current rv64 and rv32 implementation into a single rv implementation.

We updated the cmake file to match the new riscv32 arch and force
LIBC_TARGET_ARCHITECTURE to be "riscv" whenever we find "riscv32" or
"riscv64". This is required as LIBC_TARGET_ARCHITECTURE is used in the
path for several platform specific implementations.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148797