[compiler-rt][fuchsia] Preallocate a vmar for sanitizer internals (#75256)

In an effort to reduce more mmap fragmentation, allocate a large enough
vmar where we can map sanitizer internals via DoAnonymousMmap. Objects
being mapped here include asan's FakeStack, LowLevelAllocator mappings,
the primary allocator's TwoLevelMap, InternalMmapVector, StackStore, and
asan's thread internals. The vmar is large enough to hold the total size
of these objects seen in a "typical" process lifetime. If the vmar is
full, it will fallback to mapping in the root vmar.