[clang] [RISCV] make `_Complex {integer}` consistent with GCC (PR #216404)

[Folkert de Vries via cfe-commits]

folkertdev wrote:

Given that it's a GNU extension, GCC is the reference implementation right?

Most targets do already match GCC (usually by just  treating `_Complex T` as `{ T, T }` for integer types, like proposed here). Fixes to this end have now been merged to loongarch, mips, and sparc, with powerpc being the only other remaining target (that I can test) that doesn't match GCC (it also miscompiles complex float libcalls... https://github.com/llvm/llvm-project/pull/208917).

https://github.com/llvm/llvm-project/pull/216404