This ports D148280 for ASan.
On x86-64 Linux, when mmap_rnd_bits is set to 32 (the maximum ARCH_MMAP_RND_BITS_MAX),
the allocator space [kAllocatorSpace,kAllocatorSpace+kAllocatorSize)
collides with the PIE load base range (0x555555554000 upto 2**mmap_rnd_bits * pagesize away),
which can cause the allocation to fail.
Using 0x500000000000ULL as the base address avoids this problem and works with
AArch64 Linux and FreeBSD as well.
While here, change s390x to use 0x500000000000ULL as well. See D78644
discussions that the address works.