Revert "ASan: move allocator base to avoid conflict with high-entropy ASLR for x86-64 Linux"

This broke lit tests on Mac, see comment on the code review.

> Users have discovered [*] that when CONFIG_ARCH_MMAP_RND_BITS == 32,
> it will frequently conflict with ASan's allocator on x86-64 Linux, because the
> PIE program segment base address of 0x555555555554 plus an ASLR shift of up to
> ((2**32) * 4K == 0x100000000000) will sometimes exceed ASan's hardcoded
> base address of 0x600000000000. We fix this by simply moving the allocator base
> to 0x500000000000, which is below the PIE program segment base address. This is
> cleaner than trying to move it to another location that is sandwiched between
> the PIE program and library segments, because if either of those grow too large,
> it will collide with the allocator region.
>
> Note that we will never need to change this base address again (unless we want to increase
> the size of the allocator), because ASLR cannot be set above 32-bits for x86-64 Linux (the
> PIE program segment and library segments would collide with each other; see also
> ARCH_MMAP_RND_BITS_MAX in https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig).
>
> [*] see https://b.corp.google.com/issues/276925478
> and https://groups.google.com/a/google.com/g/chrome-os-gardeners/c/BbfzCP3dEeo/m/h3C_vVUxCQAJ
>
> Differential Revision: https://reviews.llvm.org/D147984

This reverts commit 6f42b97a.