[CMake] Define _FILE_OFFSET_BITS=64 on Solaris

This is the compantion patch to https://reviews.llvm.org/D64482, needed to ensure
that builds with host compilers that don't yet predefine _FILE_OFFSET_BITS=64 on
Solaris succeed by always making the host and freshly built clang consistent.

Tested on x86_64-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D64483

llvm-svn: 367304