[libc++][NFC] Remove clang-diagnostic-c++98-compat-extra-semi warnings in experimental/simd

Force semicolons or remove them in `experimental/simd`

Reviewed By: Quuxplusone, ldionne, Mordante, #libc

Spies: libcxx-commits, miyuki

Differential Revision: https://reviews.llvm.org/D117157