[Sanitizers] Solaris largefile fixes

While testing the Solaris libsanitizer port on GCC mainline, I found that
I'd messed up
the largefile checks in various ways, some of which showed as compile failures
(wrong structure sizes and member offsets), others at runtime, some of those only
on sparc as a big-endian target.

This patch fixes all of them:

- OFF_T is now correctly defined for 32-bit largefile and traditional
  environments, and 64-bit.

- The definition of __sanitizer_dirent now checks the correct conditionals.

- sanitizer_procmaps_solaris.cc undefines _FILE_OFFSET_BITS: before
  Solaris 11.4 <procfs.h> doesn't even compile with largefile support
  enabled, but the use at hand doesn't need it anyway while g++ 9 will
  define _FILE_OFFSET_BITS=64 out of the box.

- With full largefile support enabled, one needs to use e.g. mmap64
  instead of mmap; this is hidden behind macros.

With this patch I could bootstrap gcc mainline on both sparc-sun-solaris2.11 and
i386-pc-solaris2.11.  In addition, I've successfully built llvm on
i386-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D54101

llvm-svn: 346153