[MemtagABI] Clarify scope and status of the document

[yury-khrustalev]

The MemtagABI document currently states in the "Platform Standards" section:

each operating system that adopts components of this ABI specification will specify additional requirements and constraints that must be met by application code in binary form and the code-generation tools that generate such code

We should clarify that parts of the Memtag ABI that are applicable to Linux should be clarified in the SYSVABI specification:

Add this in the end:

This document does not explicitly apply to Linux, see the SYSVABI and AAPCS64 specifications for Linux implementation details.

[yury-khrustalev]

Hi @fmayer, @pcc, @zatrazz, if you have any feedback regarding this proposed change, any comments welcome, thank you!

The Linux implementation details are outlined in:

• [SYSVABI] Describe implementation details for memory tagging #382
• [AAPCS64] AAPCS64 should specify minimal requirements for setjmp and longjmp for memory tagging #383