This patch is based on https://reviews.llvm.org/D58102.
This patch adds Intel CET (Control-flow Enforcement Technology) support to
lld. The implementation follows the draft version of psABI which you can
download from https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI.
I added a comment about what Intel CET is, so I won't repeat it here, but in
summary, CET introduces a new restriction on indirect jump instructions so
that you can limit the places to which you can jump to using indirect jumps.
In order to use the feature, you need to compile source files with
-fcf-protection=full. CET is enabled only when all input files are compiled with
the flag. If you want to make sure that the feature is enabled, pass
-z force-ibt -z shstk to lld.
CET-enabled executable files have two PLT sections, ".plt" and ".plt.sec".
For the details as to why we have two sections, please read this patch's