[PATCH 0 of 1 v11] [PATCH 0 of 1] aarch64: add PAC and BTI Support]

[Bill Roberts]

This is RESEND of V11, I sent them last week but I never saw them hit the archiver, hg
reported success, try again :-p

- V11:
  - Removed unused PROLOGUE_NONLEAF
  - Fixed AC_COMPILE_IFELSE check for A and B keys
  - Used AC_COMPILE_IFELSE
  - Use  AC_COMPILE_IFELSE over AC_PREPROC_IFELSE for __ELF__ check
  - Update doc notes on PROLOGUE_cpu
  - Update regex’s in configure.ac looking for PROLOGUE macros to use -e to
    look for NONLEAF_PROLOGUE's. This prevents us from having to modify
    existing regex's.

- V10:
  - Fix copyright update

- 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