Fix incorrect macro names in arch.h

Signed-off-by: Tristan Matthews <tmatth@videolan.org>
4 jobs for arch-fixups in 1 minute and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #10982
docker
autoconf

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passed #10983
docker
fixed-point

00:00:40

passed #10985
docker
no-examples

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passed #10984
docker
no-float

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