Bug#1081943: libgl1-mesa-dri: llvmpipe exists but does not work on sparc64

[Simon McVittie <smcv@debian.org>]

Source: libgl1-mesa-dri
Version: 24.2.2-1
Severity: normal
X-Debbugs-Cc: debian-sparc@lists.debian.org
User: debian-sparc@lists.debian.org
Usertags: sparc64

To reproduce:
- install gtk4 build-dependencies on sparc64 porterbox with no access to a
  real GPU
- get gtk4 source
- edit debian/rules to remove the special case that forces use of softpipe
  on sparc64
- build and run tests

Expected result: either of these:
- llvmpipe exists, is used, and works
- llvmpipe doesn't exist and softpipe is automatically used instead

Actual result:
- all tests that use OpenGL fail with message "Target has no JIT support"

I would suggest special-casing llvmpipe (and anything else requiring LLVM
JIT: lavapipe?) to be built on most of the $(LLVM_ARCHS), but not sparc64.

    smcv