This moves the LLVM version check to the helper function
gallium_require_llvm() and uses the llvm_check_version_for() helper
instead of open conding the LLVM version check.

gallium_require_llvm is functionally the same as before, because
"enable_gallium_llvm" is only set to "yes" if the host cpu is x86:

if test "x$enable_gallium_llvm" = xauto; then
    case "$host_cpu" in
    i*86|x86_64|amd64) enable_gallium_llvm=yes;;
    esac
fi

This function is also only called now when needed.
Before this patch llvmpipe would call this as soon as LLVM is
installed. Now it only gets called by llvmpipe if gallium
LLVM is actually enabled (i.e. only on x86).

Both reasons mentioned above remove the need to check host cpu
in the gallium_require_llvm function.

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>