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

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