arm: document arm asm rules

Limiting the valid syntax to the subset supported by llvm 6.0 allows the direct use of recent Android and ios system toolchains.

@mstorsjo was were anything important added in the internal arm 32-bit assembler in llvm 5.0 or 6.0? If not, we don't have to be that restrictive.

LLVM 5.0 probably is enough. 5.0 (corresponding to NDK r17 and Xcode 9.3 iirc) added support for altmacro, in case we want to use that. Use with MS armasm from msvc tools requires gas-preprocessor in any case, and that supports altmacro.

LLVM 6.0 (corresponding to Xcode 10 and possibly NDK r18) didn't add anything new relevant for assembly. LLVM 7.0 added support for .inst for non-ELF platforms, but I doubt we'll need that.

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Agreed and merged.

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