[ASAN] Fix validation size for dirent on FreeBSD

Typically the size required to represent a dirent is stored in `d_reclen`. But
this not always the case for FreeBSD (for example, when walking a directory
over NFS).

This leads to ASAN false positives for `scandir` and similar functions. Because
ASAN uses `d_reclen` for the range to validate, it can overrun when `d_reclen` is
incorrect (too large).

This change adds `__sanitizer_dirsiz` which fixes the dirent size calculation
for FreeBSD. Other platforms continue to use `d_reclen`.

Reviewed By: vitalybuka
Differential Revision: https://reviews.llvm.org/D151583