Re-land 'ASan: move allocator base to avoid conflict with high-entrop…

…y ASLR for x86-64 Linux'

D147984 was reverted because it broke lit tests on Mac. This revision is based on D147984
but maintains the old behavior for Apple.

Note that, per the follow-up discussion with MaskRay in D147984, this patch excludes Apple
but includes other platforms (e.g., aarch64, MIPS64) and OSes (e.g., FreeBSD, S390X), not just
x86-64 Linux.

Original commit message from D147984:

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/D148280

compiler-rt/lib/asan/asan_allocator.h

@@ -143,11 +143,15 @@ typedef DefaultSizeClassMap SizeClassMap;
 const uptr kAllocatorSpace = ~(uptr)0;
 const uptr kAllocatorSize  =  0x8000000000ULL;  // 500G
 typedef DefaultSizeClassMap SizeClassMap;
-# else
+#  elif SANITIZER_APPLE
 const uptr kAllocatorSpace = 0x600000000000ULL;
 const uptr kAllocatorSize  =  0x40000000000ULL;  // 4T.
 typedef DefaultSizeClassMap SizeClassMap;
-# endif
+#  else
+const uptr kAllocatorSpace = 0x500000000000ULL;
+const uptr kAllocatorSize = 0x40000000000ULL;  // 4T.
+typedef DefaultSizeClassMap SizeClassMap;
+#  endif
 template <typename AddressSpaceViewTy>
 struct AP64 {  // Allocator64 parameters. Deliberately using a short name.
   static const uptr kSpaceBeg = kAllocatorSpace;