Control-flow Enforcement Technology (CET), published by Intel, introduces

indirect branch tracking(IBT) feature aiming to ensure the target address
of an indirect jump/call is not tampered.
When IBT is enabled, each function or target of any indirect jump/call will start
with an 'endbr32/64' instruction otherwise the program will crash during execution.
To build an application with CET enabled. we need to ensure:

  1. build the source code with "-fcf-protection=full"
  2. all the libraries linked with .o files must be CET enabled too

This patch aims to enable CET for compiler-rt builtins library, we add an option
"COMPILER_RT_ENABLE_CET" whose default value is OFF to enable CET for compiler-rt
in building time and when this option is "ON", "-fcf-protection=full" is added to
BUILTINS_CFLAG and the "endbr32/64" will be placed in the beginning of each assembly
function. We also enabled CET for crtbegin, crtend object files in this patch.

Reviewed by: MaskRay, compnerd, manojgupta, efriedma
Differential Rev...