x86inc: Add support for ELF CET properties

Automatically flag x86-64 asm object files as SHSTK-compatible.

Shadow Stack (SHSTK) is a part of Control-flow Enforcement Technology
(CET) which is a feature aimed at defending against ROP attacks by
verifying that 'call' and 'ret' instructions are correctly matched.

For well-written code this works transparently without any code changes,
as return addresses popped from the shadow stack should match return
addresses popped from the normal stack for performance reasons anyway.