Add support for an ARM specific 'y' section attribute flag to mark the section as...

Add support for an ARM specific 'y' section attribute flag to mark the section as NOREAD.

bfd/ChangeLog:
      * elf32-arm.c ((elf32_arm_special_sections): Remove catch of noread
      section using '.text.noread' pattern.

gas/ChangeLog:
      * config/obj-elf.c (obj_elf_change_section) : Allow arm section with
      SHF_ARM_NOREAD section flag.
      * config/tc-arm.h (md_elf_section_letter) : Implement this hook to
      handle letter 'y'.
     (arm_elf_section_letter) : Declare it.
      * config/tc-arm.c (arm_elf_section_letter): Handle letter 'y' to set
      SHF_ARM_NOREAD section flag.
      * doc/c-arm.texi (ARM section attribute 'y'): Document it.

gas/testsuite/ChangeLog:
      * gas/arm/section-execute-only.s: New test case.
      * gas/arm/section-execute-only.d: Expected output.

ld/testsuite/ChangeLog:
      * ld-arm/thumb1-noread-not-present-mixing-two-section.s: Add 'y'
      attribute usage.
      * ld-arm/thumb1-noread-present-one-section.s: Likewise.
      * ld-arm/thumb1-noread-present-two-section.s: Likewise.
      * ld-arm/thumb1-input-section-flag-match.s: Likewise.

binutils/ChangeLog:
      * readelf.c (get_elf_section_flags): Display y letter for section
      with SHF_ARM_NOREAD section flag in readelf section output.
      (process_section_headers): Add y letter in readelf section output
      key mapping for ARM architecture.