[compiler-rt] Fix Asan build on Android

Summary:
The dynamic shadow code is not detected correctly on Android.
The android shadow seems to start at address zero.

The bug is introduced here:
  https://reviews.llvm.org/D23363

Started here: https://build.chromium.org/p/chromium.fyi/builders/ClangToTAndroidASan/builds/4029
Likely due to an asan runtime change, filed https://llvm.org/bugs/show_bug.cgi?id=30462


From asan_mapping.h:
```
#if SANITIZER_WORDSIZE == 32
#  if SANITIZER_ANDROID
#    define SHADOW_OFFSET (0)   <<---- HERE
#  elif defined(__mips__)
```
Shadow address on android is 0.

From asan_rtl.c:
```
  if (shadow_start == 0) {
    [...]
    shadow_start = FindAvailableMemoryRange(space_size, alignment, granularity);
  }
```
We assumed that 0 is dynamic address.

On windows, the address was determined with:
```
#  elif SANITIZER_WINDOWS64
#   define SHADOW_OFFSET __asan_shadow_memory_dynamic_address
#  else
```
and __asan_shadow_memory_dynamic_address is initially zero.

Reviewers: rnk, eugenis, vitalybuka

Subscribers: kcc, tberghammer, danalbert, kubabrecka, dberris, llvm-commits, chrisha

Differential Revision: https://reviews.llvm.org/D24768

llvm-svn: 282085