Simplify regexps for commenting out unsupported directives
Implement neon register aliases

The built-in arm assembler in clang doesn't support .dn/.qn
aliases, even if Apple's normal standalone binutils-derived as
supports them.
Change a stray tab into spaces
add aarch64/arm64 architecture

Use a different comment char since the internal assembler
in Apple's clang uses '@' as syntax element.
aarch64: translate GNU as style relocations for adrp/add
aarch64: add support for .req/.unreq

clang's internal assembler does not support this directive in Xcode 5
and Xcode 5.1.