Fix diagnostic errors

Fix diagnostic errors

Fixes a number of build errors like the following
.../elf32-arm.c: In function 'elf32_arm_nabi_write_core_note':
.../elf32-arm.c:2177: error: #pragma GCC diagnostic not allowed inside functions
.../elf32-arm.c:2186: error: #pragma GCC diagnostic not allowed inside functions
See the comment in diagnostics.h.

include/
	* diagnostics.h: Comment on macro usage.
bfd/
	* elf32-arm.c (elf32_arm_nabi_write_core_note): Don't use
	DIAGNOTIC_PUSH and DIAGNOSTIC_POP unconditionally.
	* elf32-ppc.c (ppc_elf_write_core_note): Likewise.
	* elf32-s390.c (elf_s390_write_core_note): Likewise.
	* elf64-ppc.c (ppc64_elf_write_core_note): Likewise.
	* elf64-s390.c (elf_s390_write_core_note): Likewise.
	* elfxx-aarch64.c (_bfd_aarch64_elf_write_core_note): Likewise.