aarch64: Mark purecap object files with EF_AARCH64_CHERI_PURECAP
This simple patch sets the ELF header flag EF_AARCH64_CHERI_PURECAP for
purecap Morello object files, as documented in aaelf64-morello:
https://github.com/ARM-software/abi-aa/blob/main/aaelf64-morello/aaelf64-morello.rst
gas/ChangeLog:
2021-09-24 Alex Coplan <alex.coplan@arm.com>
* config/tc-aarch64.c (md_begin): Set the ELF header flag
EF_AARCH64_CHERI_PURECAP if we have the C64 extension.