[flang][safe-trampoline] Compiler fails with unimplemented `fir.emboxproc` codegen for procedure pointers

## Summary

`-fsafe-trampoline` fixes the executable-stack trampoline failure for internal procedures on hardened macOS systems, but it causes Flang to fail compiling a valid program that declares and assigns an ordinary typed procedure pointer.

This prevents projects such as MAPL from enabling `-fsafe-trampoline` as a general workaround: MAPL contains normal procedure-pointer declarations that then fail during code generation, even in sources that do not pass internal procedures as callback arguments.

## Reproducer

The below code (named `flang_safe_trampoline_procedure_pointer_reproducer.F90` on disk) declares a module-level procedure pointer, then assigns a procedure to it. It does not require MPI, external libraries, or any nonstandard extensions.

```fortran
module safe_trampoline_procedure_pointer_reproducer_mod
implicit none

abstract interface
subroutine callback()
end subroutine callback
end interface

procedure(callback), pointer :: current_callback => null()

contains

subroutine set_callback(new_callback)
procedure(callback) :: new_callback

current_callback => new_callback
end subroutine set_callback

end module safe_trampoline_procedure_pointer_reproducer_mod

program flang_safe_trampoline_procedure_pointer_reproducer
use safe_trampoline_procedure_pointer_reproducer_mod, only: set_callback
implicit none

call set_callback(no_op)

contains

subroutine no_op()
end subroutine no_op

end program flang_safe_trampoline_procedure_pointer_reproducer
```

## Commands and results

The source compiles and runs normally without `-fsafe-trampoline`:

```sh
❯ flang-23 ./flang_safe_trampoline_procedure_pointer_reproducer.F90
❯ echo $?
0
```

Adding `-fsafe-trampoline` causes a compiler failure:

```sh
❯ flang-23 -fsafe-trampoline ./flang_safe_trampoline_procedure_pointer_reproducer.F90
loc("/Users/mathomp4/Flang-Reproducers/flang_safe_trampoline_procedure_pointer_reproducer.F90":9:36): error: flang/lib/Optimizer/CodeGen/CodeGen.cpp:2786: not yet implemented: fir.emboxproc codegen
LLVM ERROR: aborting
❯ echo $?
1
```

## Environment

- Homebrew `flang-23` 23.1.1
- Target: `arm64-apple-darwin25.6.0`
- macOS 26.x on Apple Silicon

## Context

This appears distinct from the internal-procedure callback runtime failure tracked by #182813. The W^X-compliant trampoline-pool support from #183108 fixes that runtime failure when `-fsafe-trampoline` is enabled.

However, the safe-trampoline compilation path currently does not support the ordinary legal procedure-pointer construct in this reproducer. As a result, the option cannot yet be applied broadly to a project that needs the runtime trampoline fix.

## Expected behavior

The source should compile successfully with `-fsafe-trampoline`, with the same behavior as compilation without the option.

## Disclosure

GPT-5.6 Terra assisted with preparing this issue text and the minimal reproducer. The commands and results above were run and verified by the reporter.

Start with the reproducer flang_safe_trampoline_procedure_pointer_reproducer.F90 and run it with -fsafe-trampoline to reproduce the failure. Inspect flang/lib/Optimizer/CodeGen/CodeGen.cpp at the reported fir.emboxproc codegen location and trace the safe-trampoline procedure-pointer path. Done means the reproducer compiles successfully with the flag and behaves like the build without it.

Written by the indexing model from the issue text.