Remove -simplifycfg-dup-ret from CLANG_OPT_SMALL flags for clang 13

After llvm/clang 13.0.0, the -simplifycfg-dup-ret backend flag is no
longer supported. This was part of CLANG_OPT_SMALL, which is only still
used for stand/i386/boot2 and stand/i386/isoboot, to achieve the very
small binary size required. Luckily clang 13.0.0 does not need any
additional flags for this (I get 240 bytes available when building
boot2).

MFC after: 3 days