[sysvabi64] document TLSDESC resolver extension register reqs

Document the decision in the GCC mailing list thread
TLSDESC clobber ABI stability/futureproofness?
https://gcc.gnu.org/legacy-ml/gcc/2018-10/msg00112.html

TLSDESC resolver functions assume that any registers added
by an extension are caller saved for a TLSDESC call.

A brief summary:

Dynamic TLS may be lazy allocated upon the first use of a TLSDESC
resolver. This may involve calls to heap allocation functions
provided by the user, which may use registers from extensions
like SVE and SME. As the resolver function can't know what is
saved it would have to save all SVE and SME state. This would
be way more expensive than a caller save, and an older libc
written prior to the introduction of the extension would be
unaware of them so the caller has to do the save.

* The SVE and SME state is already

Author: smithp35

sysvabi64/sysvabi64.rst

@@ -14,6 +14,7 @@
 .. _AAELF64: https://github.com/ARM-software/abi-aa/releases
 .. _CPPABI64: https://developer.arm.com/docs/ihi0059/latest
 .. _GCABI: https://itanium-cxx-abi.github.io/cxx-abi/abi.html
+.. _GCCML: https://gcc.gnu.org/legacy-ml/gcc/2018-10/msg00112.html
 .. _LINUX_ABI: https://github.com/hjl-tools/linux-abi/wiki
 .. _MemTagABIELF64: https://github.com/ARM-software/abi-aa/releases
 .. _PAuthABIELF64: https://github.com/ARM-software/abi-aa/releases
@@ -243,6 +244,8 @@ This document refers to, or is referred to by, the following documents.
   +-----------------+--------------------------------------------------------------+-----------------------------------------------------------------------------+
   | GCABI_          | https://itanium-cxx-abi.github.io/cxx-abi/abi.html           | Generic C++ ABI                                                             |
   +-----------------+--------------------------------------------------------------+-----------------------------------------------------------------------------+
+  | GCCML_          | https://gcc.gnu.org/legacy-ml/gcc/2018-10/msg00112.html      | GCC Mailing list topic TLSDESC clobber ABI stability/futureproofness?       |
+  +-----------------+--------------------------------------------------------------+-----------------------------------------------------------------------------+
   | HWCAP_          | https://www.kernel.org/doc/html/latest/arm64/elf_hwcaps.html | Linux Kernel HWCAPs interface                                               |
   +-----------------+--------------------------------------------------------------+-----------------------------------------------------------------------------+
   | LINUX_ABI_      | https://github.com/hjl-tools/linux-abi/wiki                  | Linux Extensions to gABI                                                    |
@@ -2364,8 +2367,13 @@ TLS resolver functions have one argument, the address of the TLS
 descriptor, passed in ``x0``, they return the offset of the variable
 from the thread pointer in ``x0``.
 
-TLS resolver functions must save all registers that they modify with
-the exception of ``x0``, ``x1``, ``x30`` and the processor flags.
+TLS resolver functions must save all general-purpose and SIMD&FP
+registers that they modify with the exception of ``x0``, ``x1``,
+``x30`` and the processor flags.
+
+TLS resolver functions are not required to save any register added by
+an extension, such as the scalable vector registers or the SVE
+predicate registers. See `GCCML`_ for details.
 
 Example Resolver Functions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^