[memprof] Move allocator base to avoid conflict with high-entropy ASLR (#85834)

memprof often fails when ASLR entropy is too high ('sudo sysctl
vm.mmap_rnd_bits=32; ninja check-memprof'), which is the default setting
for newer versions of Ubuntu
(https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/commit/?h=hwe-6.5-next--2024.03.04-1--auto&id=6b522637c6a7dabd8530026ae933fb5ff17e877f).
This patch fixes the issue by moving the allocator base, analogously to
ASan (https://reviews.llvm.org/D148280).

Explanation from the ASan patch: when CONFIG_ARCH_MMAP_RND_BITS == 32,
it will frequently conflict with memprof's allocator, because the PIE
program segment base address of 0x555555555554 plus an ASLR shift of up
to ((2**32) * 4K == 0x100000000000) will sometimes exceed memprof'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).

Author: thurstond

compiler-rt/lib/memprof/memprof_allocator.h

@@ -46,7 +46,11 @@ struct MemprofMapUnmapCallback {
   void OnUnmap(uptr p, uptr size) const;
 };
 
+#if SANITIZER_APPLE
 constexpr uptr kAllocatorSpace = 0x600000000000ULL;
+#else
+constexpr uptr kAllocatorSpace = 0x500000000000ULL;
+#endif
 constexpr uptr kAllocatorSize = 0x40000000000ULL; // 4T.
 typedef DefaultSizeClassMap SizeClassMap;
 template <typename AddressSpaceViewTy>