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Intel Control-flow Enforcement Technology (CET):

https://software.intel.com/en-us/articles/intel-sdm

contains shadow stack (SHSTK) and indirect branch tracking (IBT).  When
CET is enabled, ELF object files must be marked with .note.gnu.property
section.  Also when IBT is enabled, all indirect branch targets must
start with ENDBR instruction.

This patch adds ASM_X86_ENDBR and the x86 CET marker to config.m4.in when
CET is enabled.  It updates PROLOGUE with ASM_X86_ENDBR.

Tested on CET machine with i686 and x86-64.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>