rustc_codegen_llvm: Adjust RISC-V inline assembly's clobber list

Despite that the `fflags` register (representing floating point
exception flags) is stated as a flag register in the reference, it's not
in the default clobber list of the RISC-V inline assembly and it would
be better to fix it.

compiler/rustc_codegen_llvm/src/asm.rs

@@ -240,6 +240,7 @@ impl<'ll, 'tcx> AsmBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 }
                 InlineAsmArch::RiscV32 | InlineAsmArch::RiscV64 => {
                     constraints.extend_from_slice(&[
+                        "~{fflags}".to_string(),
                         "~{vtype}".to_string(),
                         "~{vl}".to_string(),
                         "~{vxsat}".to_string(),