[asan] Add the FreeBSD AArch64 shadow offset

[andrew]

Summary

As with 64 bit x86 use an offset in middle of the address space scaled up to work with the full 48 bit space.

[MaskRay]

Please add some description about what tests previously failed now work.

[andrew]

There is no change in any tests passing as no asan tests are run on FreeBSD AArch64. This is compiler-rt part of the change that is needed to support asan there. There is also D125756 for the common sanitizer parts, and an as yet unsubmitted changes to add the offset to llvm and enable the sanitizers in clang.

With these the only failing asan test is SanitizerCommon-asan-aarch64-FreeBSD :: print-stack-trace.cpp. This seems to be a broken test as it inlines a function but assumes the caller and inlined callee will have different addresses.

compiler-rt/lib/asan/asan_mapping.h

 #  else
 #    define ASAN_SHADOW_OFFSET_CONST 0x20000000
 #  endif
 #else
 #  if SANITIZER_IOS
 #    define ASAN_SHADOW_OFFSET_DYNAMIC
 #  elif SANITIZER_APPLE && defined(__aarch64__)
 #    define ASAN_SHADOW_OFFSET_DYNAMIC
+#  elif SANITIZER_FREEBSD && defined(__aarch64__)
+#    define ASAN_SHADOW_OFFSET_CONST 0x0000800000000000
 #  elif SANITIZER_RISCV64
 #    define ASAN_SHADOW_OFFSET_CONST 0x0000000d55550000
 #  elif defined(__aarch64__)
 #    define ASAN_SHADOW_OFFSET_CONST 0x0000001000000000
 #  elif defined(__powerpc64__)
 #    define ASAN_SHADOW_OFFSET_CONST 0x0000100000000000
 #  elif defined(__s390x__)
 #    define ASAN_SHADOW_OFFSET_CONST 0x0010000000000000