liblzma: Fix building with NVHPC (NVIDIA HPC SDK).

liblzma: Fix building with NVHPC (NVIDIA HPC SDK).

NVHPC compiler has several issues that make it impossible to
build liblzma:
  - the compiler cannot handle unions that contain pointers that
    are not the first members;
  - the compiler fails to produce valid code for delta_decode if the
    vectorization is enabled, which results in failed tests.

This introduces NVHPC-specific workarounds that address the issues.

(This commit was contributed under 0BSD but the author confirmed
that it is fine to backport it to the public domain branches. See
https://github.com/tukaani-project/xz/pull/90#issuecomment-2100185936
and the next two messages.)

(cherry picked from commit 096bc0e3f8fb4bfc4d2f3f64a7f219401ffb4c31)