Remove use of -fno-asm-mangling flag; fixes #5134

The flag no longer exists, but we always behave as if it had been used.

compiler/main/StaticFlagParser.hs

@@ -210,7 +210,6 @@ unregFlags :: [Located String]
 unregFlags = map (mkGeneralLocated "in unregFlags")
    [ "-optc-DNO_REGS"
    , "-optc-DUSE_MINIINTERPRETER"
-   , "-fno-asm-mangling"
    , "-funregisterised" ]
 
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