FreeBSD: fix check for mmap flags

FreeBSD: fix check for mmap flags

On FreeBSD, mmap also has MAP_STACK and MAP_GUARD that can
be mapped without a backing file referred to by fd.

As a result during ld.so startup and thread creation mmap for
stacks was failing. So no guest could be load and execute,
with errors like

ld-elf.so.1: /home/paulf/scratch/valgrind_nightly/nightly/valgrind-new/.in_place/vgpreload_core-amd64-freebsd.so: mmap of entire address space failed: Bad file descriptor