libm: fix truncl when long double has same size as double

[Cynerd]

Summary

The truncl implementation expects 80 bit little endian representation of the floating point number. That is valid for x86 but not for example arm. On arm long double is the same as double.

This was discovered with newer version of GCC (15.2.0) that started emitting warning:

  libm/lib_truncl.c: In function 'truncl':
  libm/lib_truncl.c:68:14: error: 'u.i.se' is used uninitialized [-Werror=uninitialized]
     68 |   int e = u.i.se & 0x7fff;
        |           ~~~^~~
  libm/lib_truncl.c:63:17: note: 'u' declared here
     63 |   union ldshape u =
        |                 ^
  libm/lib_truncl.c:69:14: error: 'u.i.se' is used uninitialized [-Werror=uninitialized]
     69 |   int s = u.i.se >> 15;
        |           ~~~^~~
  libm/lib_truncl.c:63:17: note: 'u' declared here
     63 |   union ldshape u =
        |                 ^
  libm/lib_truncl.c:63:17: error: 'u.i.se' is used uninitialized [-Werror=uninitialized]

Impact

This is technically a fix for truncl on platforms that have long double of the same size as double, as the logic was correct only for 80 bit float.

Testing

The testing was performed only by compilation, as the implementation just redirects to the trunc instead. The warning disappeared with the changes made in this PR.

[Cynerd]

I just tested it with x86 and the condition is wrong because https://github.com/apache/nuttx/blob/master/include/nuttx/lib/float.h#L67 ... ups