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).

(cherry picked from commit 22b8ab15c41a9efac201691b40e961b83698aa9c)