[llvm] [RISCV][MC] Emit `x8` as `fp` instead of `s0` (PR #135500)

[Sam Elliott via llvm-commits]

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@@ -34,6 +34,10 @@ static cl::opt<bool>
               cl::desc("Disable the emission of assembler pseudo instructions"),
               cl::init(false), cl::Hidden);
 
+static cl::opt<bool> EmitX8AsFP("riscv-emit-x8-as-fp",
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lenary wrote:

Can we move this to a `-M` option, rather than a hidden `cl`? I think we're trying to move all uses of `riscv-no-aliases` to the equivalent `-M` option.

https://github.com/llvm/llvm-project/pull/135500