[clang] [RISCV] Add -march=unset to cancel and ignore a previous -march. (PR #148321)

[Fangrui Song via cfe-commits]

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@@ -403,6 +403,11 @@
 // MCPU-MARCH: "-nostdsysteminc" "-target-cpu" "sifive-e31" "-target-feature" "+m" "-target-feature" "+c"
 // MCPU-MARCH: "-target-abi" "ilp32"
 
+// march=unset erases previous march
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MaskRay wrote:

`/// -march=unset erases previous -march`.

Always include `-` when referring to a `-` option.

https://github.com/llvm/llvm-project/pull/148321