Make wamrc normalize "arm64" to "aarch64v8". Previously the only way to
make the "arm64" target was to not specify a target on 64 bit arm-based
mac builds. Now arm64 and aarch64v8 are treated as the same.
Make aot_loader accept "aarch64v8" on arm-based apple (as well as
accepting legacy "arm64" based aot targets).
This also removes __APPLE__ and __MACH__ from the block that defaults
size_level to 1 since it doesn't seem to be supported for aarch64:
`LLVM ERROR: Only small, tiny and large code models are allowed on AArch64`
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