New package: libretro-mupen64plus-nx-20260520

[Rutpiv]

Testing the changes

• I tested the changes in this PR: YES

Local build testing

• I built this PR locally for my native architecture (x86_64)
• I built this PR locally for these architectures using specific masterdirs:
  • x86_64-musl
  • i686
• I built this PR locally for these architectures (crossbuilds):
  • aarch64
  • aarch64-musl
  • armv7l
  • armv7l-musl
  • armv6l
  • armv6l-musl

---

Replaces the orphaned libretro-mupen64plus with libretro-mupen64plus-nx, based on the actively maintained successor repo (libretro/mupen64plus-libretro-nx). The original upstream is no longer maintained and no longer builds with current toolchains; the failures predate the nasm update and were discovered while validating dependent packages in #57682.

• The core's .so filename changes with the new upstream. Since RetroArch references cores by filename, a transitional dummy package is kept and an INSTALL.msg notifies users about updating their core config.
• ParaLLEl RDP/RSP, LLE, and threaded angrylion are enabled on x86_64* and aarch64*, where the Vulkan-based renderer is supported.
• ARM targets default to GLES2.
• Dynarec is selected via ARCH.

[Rutpiv]

The musl CI failure was caused by a missing patch I forgot to include in the original push: the vendored parallel-rdp copy lacks the stdint.h include fixed upstream in Granite (9e78ed6c, "util: fix the musl build"). My original build matrix predates this change, so I'm re-running it against the final tree; will update here once it completes.