[Bug 286593] devel/llvm15: wrong stack guard check on armv7

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=286593

            Bug ID: 286593
           Summary: devel/llvm15: wrong stack guard check on armv7
           Product: Ports & Packages
           Version: Latest
          Hardware: arm
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: brooks@FreeBSD.org
          Reporter: fuz@FreeBSD.org
                CC: Andrew@FreeBSD.org
          Assignee: brooks@FreeBSD.org
             Flags: maintainer-feedback?(brooks@FreeBSD.org)

Debugging a crash caused during the compilation of emulators/uae on armv7
FreeBSD 14.2, it appears that devel/llvm15 generates wrong code for stack
checks.  The cc we ship with base generates this sort of code in main:

00000000 <main>:
       0: e92d4ff0      push    {r4, r5, r6, r7, r8, r9, r10, r11, lr}
       4: e28db01c      add     r11, sp, #28
       8: e24dd004      sub     sp, sp, #4
       c: ed2d8b04      vpush   {d8, d9}
      10: e24dde19      sub     sp, sp, #400
      14: e3000000      movw    r0, #0x0
                        00000014:  R_ARM_MOVW_ABS_NC    __stack_chk_guard
      18: e3400000      movt    r0, #0x0
                        00000018:  R_ARM_MOVT_ABS       __stack_chk_guard
      1c: e5900000      ldr     r0, [r0]
      20: e50b0038      str     r0, [r11, #-0x38]

i.e. the stack check guard token is loaded from __stack_chk_guard and then
saved on the stack.  When compiling the same file with devel/llvm15, I get this
however:

00000000 <main>:
       0: e92d4ff0      push    {r4, r5, r6, r7, r8, r9, r10, r11, lr}
       4: e28db01c      add     r11, sp, #28
       8: e24dd004      sub     sp, sp, #4
       c: ed2d8b04      vpush   {d8, d9}
      10: e24ddf72      sub     sp, sp, #456
      14: e3000000      movw    r0, #0x0
                        00000014:  R_ARM_MOVW_ABS_NC    __stack_chk_guard
      18: e3400000      movt    r0, #0x0
                        00000018:  R_ARM_MOVT_ABS       __stack_chk_guard
      1c: e5900000      ldr     r0, [r0]
      20: e5900000      ldr     r0, [r0]
      24: e50b0038      str     r0, [r11, #-0x38]

This code differs in that there is an extra dereference of __stack_chk_guard. 
As __stack_chk_guard holds a random number, this dereference is unlikely to go
right, crashing the program.

Please investigate.

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