[MC] Add 'G' to augmentation string for MTE instrumented functions

This was agreed on in
https://lists.llvm.org/pipermail/llvm-dev/2020-May/141345.html

The thread proposed two options
* add a character to augmentation string and handle in libuwind
* use a separate personality function.

It was determined that this is the simpler and better option.

This is part of ARM's Aarch64 ABI:
https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#id22

The next step after this is teaching libunwind to untag when this
augmentation character is set.

Reviewed By: MaskRay, eugenis

Differential Revision: https://reviews.llvm.org/D127007