Wire ARCH_AARCH64 into to_executable_fmt for exe and exe-only formats

Summary

Msf::Util::EXE::ClassMethods#to_executable_fmt is missing ARCH_AARCH64
branches in its 'exe' and 'exe-only' cases. The helper
to_winaarch64pe (in lib/msf/util/exe/windows/aarch64.rb) is fully
implemented and the corresponding template
(data/templates/template_aarch64_windows.exe) ships with the
framework, but neither is reachable from msfvenom today because the
format dispatcher silently falls through when arch is AArch64 and
returns nil.

This means the existing windows/aarch64/exec payload — and any future
Windows AArch64 payload — cannot be wrapped into a PE via msfvenom:

$ ./msfvenom -p windows/aarch64/exec CMD=calc.exe -f exe -o exec.exe
[-] No platform was selected, choosing Msf::Module::Platform::Windows from the payload
[-] No arch selected, selecting arch: aarch64 from the payload
No encoder specified, outputting raw payload
Payload size: 360 bytes
Error: The payload could not be generated, check options

Fix

Add the missing when ARCH_AARCH64 branches to both 'exe' and
'exe-only' cases, dispatching to the already-existing
to_winaarch64pe helper:

when 'exe'
  case arch
  when ARCH_X86, nil
    to_win32pe(framework, code, exeopts)
  when ARCH_X64
    to_win64pe(framework, code, exeopts)
  when ARCH_AARCH64                         # <-- added
    to_winaarch64pe(framework, code, exeopts)
  end

Same two-line addition for 'exe-only'. No other code paths touched.

Verification

Before this PR:

$ ./msfvenom -p windows/aarch64/exec CMD=calc.exe -f exe -o exec.exe
Error: The payload could not be generated, check options

After this PR:

$ ./msfvenom -p windows/aarch64/exec CMD=calc.exe -f exe -o exec.exe
[*] Payload size: 360 bytes
[*] Final size of exe file: 6656 bytes
[*] Saved as: exec.exe

$ file exec.exe
exec.exe: PE32+ executable (console) Aarch64, for MS Windows

The resulting PE was executed on Windows 11 ARM64 (build 26200) and
correctly launched calc.exe.

-f raw, -f c, -f hex, and -f bin already worked because they do
not go through the EXE wrapper. This PR makes -f exe and -f exe-only
catch up to the same support level.

Test plan

• ./msfvenom -p windows/aarch64/exec CMD=calc.exe -f exe -o exec.exe succeeds
• file exec.exe reports a PE32+ AArch64 console executable
• exec.exe runs and launches calc.exe on a Windows 11 ARM64 machine
• ./msfvenom -p windows/aarch64/exec CMD=calc.exe -f exe-only -o exec_only.exe succeeds
• Existing x86 and x64 -f exe / -f exe-only paths are unaffected (regression check)