configure.ac: Require LLVM for r300 only on x86 and x86_64

configure.ac: Require LLVM for r300 only on x86 and x86_64

b3119a3 introduced a strict LLVM requirement for r300 on all
architectures and thus configure fails on architectures where LLVM is
not available or buggy.

r300 doesn't strictly require LLVM, but for performance reasons we
highly recommend LLVM usage. So require it at least on x86 and x86_64
architectures as we have done before b3119a3.

Fixes: b3119a3 ("configure.ac: Check gallium LLVM version in gallium_require_llvm")
Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>