[DFSan] Handle mmap() calls before interceptors are installed.

InitializeInterceptors() calls dlsym(), which calls calloc().  Depending
on the allocator implementation, calloc() may invoke mmap(), which
results in a segfault since REAL(mmap) is still being resolved.

We fix this by doing a direct syscall if interceptors haven't been fully
resolved yet.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D86168