[clang] [clang][RISCV] Remove some of the bits added with RISC-V big endian support (PR #192903)

[Djordje Todorovic via cfe-commits]

djtodoro wrote:

Thanks for this!

Can we add some driver tests to ensure that the BE support for FreeBSD is not intentional (atm at least)?

https://github.com/llvm/llvm-project/pull/192903