[msan] Use SizeClassAllocator64 for AArch64

Now that D137666 requires 48-bit VMA for AArch64, we can switch to
SizeClassAllocator64 for a slightly more efficient allocator
(asan/lsan already switched by default).

It seems that we can pick kSpaceBeg = 0xE00000000000ULL to support both Linux
("app-15") and FreeBSD ("high memory").

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D140792