diffutils: update to 3.12.

[vincele]

Testing the changes

• I tested the changes in this PR: briefly

A quick diff & cmp on a bunch of files ran OK.

And xbp-src check diffutils output:

[...]
============================================================================
Testsuite summary for GNU diffutils 3.12
============================================================================
# TOTAL: 344
# PASS:  279
# SKIP:  65
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

Local build testing

• I built this PR locally for my native architecture, (x86_64-musl)
• I built this PR locally for these architectures (if supported. mark crossbuilds):
  • NOPE, tried aarch64 (crossbuild) but that fails

[vincele]

@Gottox is that good for you ?

EDIT: obviously not, with all those CI failures... That are all the same: configure: error: cannot run test program while cross compiling

Will test fix from: https://savannah.gnu.org/bugs/?66978

EDIT-2: the workaround works that failure around, but do we want that ?

This is coming from the updated gnulib included in diffutils 3.12, that breaks cross-compilation, see upstream bug report there:
https://lists.gnu.org/archive/html/bug-gnulib/2025-04/msg00055.html

Should I add the info in the commit msg ? a comment in the template ?