Sync x86_64 fcontext fixes

[kn1g78]

This fixes the TLS stack protector save location, removes duplicated shadow stack handling, and adds CFI information for the make_fcontext trampoline.

[kn1g78]

The intent of this PR is small: sync the upstream Boost.Context fixes relevant
to x86_64 Fiber/fcontext assembly.

The diff is larger because Zend/asm is a bundled upstream copy. Partial
cherry-picks fail Verify Bundled Files, so this had to be turned into a full
Boost.Context bundle sync from 1.86.0 to 1.91.0 by updating
boost-context.sh and
regenerating
Zend/asm. Most of the added lines are therefore upstream sync churn, not
new hand-written logic.

For reference, this was the original targeted x86_64 diff before converting it
into a full bundled sync:

diff --git a/Zend/asm/jump_x86_64_sysv_elf_gas.S b/Zend/asm/jump_x86_64_sysv_elf_gas.S
@@ -67,14 +67,6 @@ jump_fcontext:
      movq  %rbx, 0x30(%rsp)  /* save RBX */
      movq  %rbp, 0x38(%rsp)  /* save RBP */

-#if BOOST_CONTEXT_SHADOW_STACK
-    /* grow the stack to reserve space for shadow stack pointer(SSP) */
-    leaq  -0x8(%rsp), %rsp
-    /* read the current SSP and store it */
-    rdsspq  %rcx
-    movq  %rcx, (%rsp)
-#endif
-
  #if BOOST_CONTEXT_SHADOW_STACK
      /* grow the stack to reserve space for shadow stack pointer(SSP) */
      leaq  -0x8(%rsp), %rsp

diff --git a/Zend/asm/make_x86_64_sysv_elf_gas.S b/Zend/asm/make_x86_64_sysv_elf_gas.S
@@ -76,7 +76,7 @@ make_fcontext:
  #if defined(BOOST_CONTEXT_TLS_STACK_PROTECTOR)
      /* save stack guard */
      movq  %fs:0x28, %rcx    /* read stack guard from TLS record */
-    movq  %rcx, 0x8(%rsp)   /* save stack guard */
+    movq  %rcx, 0x8(%rax)   /* save stack guard */
  #endif

@@ -91,35 +91,6 @@ make_fcontext:
      /* will be entered after context-function returns */
      movq  %rcx, 0x38(%rax)

-#if BOOST_CONTEXT_SHADOW_STACK
-    /* Populate the shadow stack and normal stack */
-    /* get original SSP */
-    rdsspq  %r8
-    /* restore new shadow stack */
-    rstorssp  -0x8(%r9)
-    /* save the restore token on the original shadow stack */
-    saveprevssp
-    /* push the address of "jmp trampoline" to the new shadow stack */
-    /* as well as the stack */
-    call  1f
-    jmp  trampoline
-1:
-    /* save address of "jmp trampoline" as return-address */
-    /* for context-function */
-    pop 0x38(%rax)
-    /* Get the new SSP.  */
-    rdsspq  %r9
-    /* restore original shadow stack */
-    rstorssp  -0x8(%r8)
-    /* save the restore token on the new shadow stack.  */
-    saveprevssp
-
-    /* reserve space for the new SSP */
-    leaq  -0x8(%rax), %rax
-    /* save the new SSP to this fcontext */
-    movq  %r9, (%rax)
-#endif
-
  #if BOOST_CONTEXT_SHADOW_STACK
      /* Populate the shadow stack */

@@ -161,6 +132,8 @@ make_fcontext:
      ret /* return pointer to context-data */

  trampoline:
+    .cfi_startproc
+    .cfi_undefined rip
      /* store return address on stack */
      /* fix stack alignment */
      _CET_ENDBR
@@ -175,6 +148,7 @@ trampoline:
  #endif
      /* jump to context-function */
      jmp *%rbx
+    .cfi_endproc