devel/llvm*: Fix BE_NATIVE on aarch64

Fix two bugs in native-only backend support (BE_NATIVE). First, ARCH
is aarch64 not arm64. Second, consistently spell AArch64 not AAarch64
(extra 'a').

Because this only effects aarch64 in a non-default configuration, skip
the PORTREVISION bump that is technically required to reduce excess
heating in common cases.

Reported by: Mark Millard