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

[Djordje Todorovic via cfe-commits]

djtodoro wrote:

> > Can we add some driver tests to ensure that the BE support for FreeBSD is not intentional (atm at least)?
> 
> Suggestions of how to test as opposed to the other way around.

For example, we could add one test in `clang/test/Preprocessor/`, something like:

```
// RUN: %clang -target riscv32be-unknown-freebsd -dM -E -x c /dev/null 2>&1 | FileCheck %s
// CHECK-NOT: __FreeBSD__
```

And/or, in `clang/test/Driver/`, something like:

```
// RUN: %clang -target riscv32be-unknown-freebsd -### -c %s 2>&1 | FileCheck %s --check-prefix=RV32BE
// RV32BE-NOT: elf32briscv
```

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