Add support for Indirect Branch Tracking (IBT)

Indirect Branch Tracking(IBT) is part of Intel's Control-Flow
Enforcement Technology(CET). IBT is hardware based, forward edge
Control-Flow-Integrity mechanism where any indirect CALL/JMP must
target an ENDBR instruction or suffer #CP.

This commit inserts endbr64 instruction in assembly to support IBT.

src/asm/jump_x86_64_sysv_elf_gas.S

@@ -23,13 +23,18 @@
  *  ----------------------------------------------------------------------------------  *
  *                                                                                      *
  ****************************************************************************************/
-
+# if defined __CET__
+#  include <cet.h>
+# else
+#  define _CET_ENDBR
+# endif
 .file "jump_x86_64_sysv_elf_gas.S"
 .text
 .globl jump_fcontext
 .type jump_fcontext,@function
 .align 16
 jump_fcontext:
+    _CET_ENDBR
     leaq  -0x38(%rsp), %rsp /* prepare stack */
 
 #if !defined(BOOST_USE_TSX)

src/asm/make_x86_64_sysv_elf_gas.S

@@ -23,13 +23,18 @@
  *  ----------------------------------------------------------------------------------  *
  *                                                                                      *
  ****************************************************************************************/
-
+# if defined __CET__
+#  include <cet.h>
+# else
+#  define _CET_ENDBR
+# endif
 .file "make_x86_64_sysv_elf_gas.S"
 .text
 .globl make_fcontext
 .type make_fcontext,@function
 .align 16
 make_fcontext:
+    _CET_ENDBR
     /* first arg of make_fcontext() == top of context-stack */
     movq  %rdi, %rax
 
@@ -64,13 +69,15 @@ make_fcontext:
     ret /* return pointer to context-data */
 
 trampoline:
+    _CET_ENDBR
     /* store return address on stack */
     /* fix stack alignment */
     push %rbp
     /* jump to context-function */
     jmp *%rbx
 
 finish:
+    _CET_ENDBR
     /* exit code is zero */
     xorq  %rdi, %rdi
     /* exit application */

src/asm/ontop_x86_64_sysv_elf_gas.S

@@ -23,13 +23,18 @@
  *  ----------------------------------------------------------------------------------  *
  *                                                                                      *
  ****************************************************************************************/
-
+# if defined __CET__
+#  include <cet.h>
+# else
+#  define _CET_ENDBR
+# endif
 .file "ontop_x86_64_sysv_elf_gas.S"
 .text
 .globl ontop_fcontext
 .type ontop_fcontext,@function
 .align 16
 ontop_fcontext:
+    _CET_ENDBR
     /* preserve ontop-function in R8 */
     movq  %rdx, %r8