Reland [sanitizer] Support Intel CET

1. Include <cet.h> in sanitizer_common/sanitizer_asm.h, if it exists, to
mark Intel CET support when Intel CET is enabled.
2. Define _CET_ENDBR as empty if it isn't defined.
3. Add _CET_ENDBR to function entries in assembly codes so that ENDBR
instruction will be generated when Intel CET is enabled.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D111185