Use union to bitcast int to float

[zeux]

This fixes GCC's strict-aliasing warning when building basisu with gcc
9.3 in release:

/mnt/c/work/basis_universal/encoder/cppspmd_sse.h: In function ‘float cppspmd_sse41::extractf_ps_x(const __m128&)’:
/mnt/c/work/basis_universal/encoder/cppspmd_sse.h:219:98: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  219 | CPPSPMD_FORCE_INLINE float extractf_ps_x(const __m128& vec) { int v = extract_ps_x(vec); return *(const float*)&v; }
      |                                                                                                  ^~~~~~~~~~~~~~~~

While technically union bitcast still violates strict aliasing,
both clang and gcc support this as an unofficial extension.

[zeux]

@richgel999 Let me know if this needs any revisions to be considered for merging