Support x86-64 5-level paging for ASan (and possibly other sanitizers)

[thurstond]

The failure mode for ASan is explained by the test added in llvm/llvm-project@6f66b12:

    [sanitizer-common] Expand testing for 56-bit address spaces
    
    This patch adds mmap_56bit_test.c, which attempts to deliberately
    mmap above 2^47. This currently exposes a failure on ASan on
    systems with 56-bit user address spaces, because it allows an mmap
    above 2^47 but the shadow mapping is only set up for a 2^47 user address
    space.
    ...
    Differential Revision: https://reviews.llvm.org/D149176

Why it fails

ASan's dynamic shadow algorithm automatically figures out the address space size:

// compiler-rt/lib/asan/asan_rtl.cpp
static void InitializeHighMemEnd() {
#if !ASAN_FIXED_MAPPING
  kHighMemEnd = GetMaxUserVirtualAddress();

edit:
but the MaxVirtualAddress is hardcoded in compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

Suggested fixes for ASan

• (better) correctly figure out the max user virtual address (such as by deliberately trying to map above 47-bit). The dynamic shadow algorithm will then fix itself, and ASan should work with the full address space.
• (worse) change the mmap interceptor to prevent it from mapping above 47-bit (i.e., pretend it is a 47-bit system).

Other sanitizers

Similar issues apply to most other sanitizers; those are harder to fix, because they mostly use handcrafted mappings rather than dynamic shadows.