[PATCH 0 of 1] RESEND: aarch64: PAC and BTI Support

[bill.roberts at foss.arm.com]

Hello,

This is a resend of these patches rebased onto current master:
  - https://gmplib.org/list-archives/gmp-devel/2025-November/006330.html

The only changes since then, are that the GMP FSF agreement is complete and
have hg email working, so the patches won't be in git format anymore. 

See Commit Message for More Details

- V9:
  - drop the separate patches in the series using AC_PROG_SED|GREP
  - use env vars for the test script
  - make the test skip if readelf is not available for the test
  - Make the test script POSIX shell only

- V8:
  - fix commit message: "echo message | sed -i s/sed/grep"

- V7:
  - Drop GMP_GET_MACRO_VALUE
  - Drop GMP_CHECK_PROG
  - configure.ac already uses sed and grep, hardcoded, so add checks for them
    and use the variables over the direct command
  - Bolster documentation with links to the Arm ARM, ABI and Blog documentations
    - Make some clarifications in the commit message as well
  - Add NONLEAF_PROLOGUE instead of calling PROLOGUE(xxx, true)
    - naming convention follows MULFUNC_PROLOGUE naming convention
    - Update the pattern matching for determining if generic C or asm should be used
    - Requires EGREP for automatic configuration of assembly routines over generic
      C implementations
  - Add License to t-arm64_elf_check.sh
  - Detect sed and grep using autoconf and use the proper SED and GREP variables
    respectively to invoke them vs hardcoding it
  - Drop erroneous Makefile.am formatting change

- V6:
  - Fix commit message wording
  - Fix double use of sign in commit message when verify was intended
  - Drop spurious whitespace only change in configure.ac
  - comment in tests/mpn/t-arm64_bti.c why we return 0 if the symbol is not found
  - Fix table whitespace in tests/mpn/t-arm64_bti.c
  - Use declare -g over eval in tests/mpn/t-arm64_elf_check.sh
  - Quote rc calculation and exit in tests/mpn/t-arm64_elf_check.sh
  - Fix typo in tests/mpn/t-arm64_elf_check.sh "binary ast he ONLY" -> "binary as the ONLY"
 
- V5:
  - Fix cross compilation support
- V4:
  - Fix typos in bash script comments
- V3:
  - Use divert(1) to add GNU Notes to ELF files to avoid
    needing the macro in each file.
- V2:
  - Add tests
- V1:
  - Initial Commit