fix: unblock SM8750 Mesa and GCC 15 package builds

[ddordie]

What changed

• switch Mesa from the unpublished 26.2.0 final archive to the published 26.2.0-rc1 tarball and verified SHA256
• remove the duplicate pre_configure_host() definition in binutils so the existing host flag cleanup is not shadowed
• pass the NSS GCC compatibility flag through XCFLAGS, which is consumed by the legacy coreconf build system

Why

The previous Mesa URL returned 404 because Mesa 26.2.0 final has not been released yet. The duplicate binutils hook replaced the package's existing cleanup hook at shell load time. NSS coreconf does not reliably consume an environment-only CFLAGS override and provides XCFLAGS for externally supplied compiler flags.

Together these issues can stop the ROCKNIX build before the SM8750/A830 runtime path is reached.

Validation

• confirmed the branch differs from optimize-steam-install in only the three intended package files
• verified there is exactly one binutils pre_configure_host() hook and no remaining -std=gnu99 override
• verified the NSS workaround is supplied through XCFLAGS
• verified https://archive.mesa3d.org/mesa-26.2.0-rc1.tar.xz returns HTTP 200

Follow-up

This PR intentionally does not alter the SM8750 kernel patch stack. The remaining A8xx GX GDSC/IFPC series should be reviewed separately after the image can build successfully.