mesa: gate NPU drivers on 64-bit targets only

ethosu fails to build entirely, rocket does a bunch of pointer-to-int64
and back casts that will be wildly unsafe on 32-bit.

pkgs/development/libraries/mesa/default.nix

@@ -51,7 +51,6 @@
     "asahi" # Apple AGX
     "crocus" # Intel legacy
     "d3d12" # WSL emulated GPU (aka Dozen)
-    "ethosu" # ARM Ethos NPU
     "etnaviv" # Vivante GPU designs (mostly NXP/Marvell SoCs)
     "freedreno" # Qualcomm Adreno (all Qualcomm SoCs)
     "i915" # Intel extra legacy
@@ -63,7 +62,6 @@
     "r300" # very old AMD
     "r600" # less old AMD
     "radeonsi" # new AMD (GCN+)
-    "rocket" # Rockchip NPU
     "softpipe" # older software renderer
     "svga" # VMWare virtualized GPU
     "tegra" # Nvidia Tegra SoCs
@@ -71,6 +69,10 @@
     "vc4" # Broadcom VC4 (Raspberry Pi 0-3)
     "virgl" # QEMU virtualized GPU (aka VirGL)
     "zink" # generic OpenGL over Vulkan, experimental
+  ]
+  ++ lib.optionals stdenv.hostPlatform.is64bit [
+    "ethosu" # ARM Ethos NPU, does not build on 32-bit
+    "rocket" # Rockchip NPU, probably horribly broken on 32-bit
   ],
   vulkanDrivers ? [
     "amd" # AMD (aka RADV)