x86: Add indirect branch tracking support (#540)

x86: Add indirect branch tracking support (#540)

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.  When Intel CET is enabled, include <cet.h> in assembly codes
to mark Intel CET support.

Also when IBT is enabled, all indirect branch targets must start with
ENDBR instruction and notrack prefix can be used to disable IBT on
indirect branch.  <cet.h> defines _CET_ENDBR which can be used in
assembly codes for ENDBR instruction.  If <cet.h> isn't included,
define _CET_ENDBR as empty so that _CET_ENDBR can be used in assembly
codes.

Trampoline must be enlarged to add ENDBR instruction unconditionally,
which is NOP on non-CET processors.  This is required regardless if
libffi is enabled with CET since libffi.so will be marked in legacy
bitmap, but trampoline won't.  Update library version for larger
FFI_TRAMPOLINE_SIZE.

This fixed:

https://github.com/libffi/libffi/issues/474

Tested with

$ CC="gcc -Wl,-z,cet-report=error -fcf-protection" CXX="g++ -Wl,-z,cet-report=error -fcf-protection" .../configure

on Linux CET machines in i686, x32 and x86-64 modes.