[sanitizer] Relax the restriction on SizeClassAllocator64::kAllocatorSize

Commit 278ccdac says that kAllocatorSize
must be >= (1<<32), but this is not accurate. This static_assert causes 128GiB
kAllocatorSize to be unable to select DefaultSizeClassMap (kRegionSize is
1<<31).

Relax the restriction to be able to satisfy the largest size class. This allows
DefaultSizeClassMap to be usable with 128GiB kAllocatorSize, with
check-{asan,lsan,sanitizer} passing.

Reviewed By: #sanitizers, vitalybuka, kstoimenov

Differential Revision: https://reviews.llvm.org/D153664