[asan] Add missing #include of sanitizer_platform.h

[mcgrathr]

Summary

The "asan/asan_mapping.h" header relies on sanitizer_platform.h
macros, but doesn't directly include the header. All the existing
uses until recently happened to be in places where some other header
had indirectly included sanitizer_platform.h first. The addition of
asan_rtl_x86_64.S was the first place to use "asan/asan_mapping.h"
alone. It so happens that its uses of the macros make having no
macros defined equivalent to SANITIZER_LINUX, so this did not affect
Linux builds. But the assembly constants in asan_rtl_x86_64.S were
wrong for Fuchsia when SANITIZER_FUCHSIA was not properly defined.

compiler-rt/lib/asan/asan_mapping.h

 //===-- asan_mapping.h ------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file is a part of AddressSanitizer, an address sanity checker.
 //
 // Defines ASan memory mapping.
 //===----------------------------------------------------------------------===//
 #ifndef ASAN_MAPPING_H
 #define ASAN_MAPPING_H
 
+#include "sanitizer_common/sanitizer_platform.h"
+
 // The full explanation of the memory mapping could be found here:
 // https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm
 //
 // Typical shadow mapping on Linux/x86_64 with SHADOW_OFFSET == 0x00007fff8000:
 // || `[0x10007fff8000, 0x7fffffffffff]` || HighMem    ||
 // || `[0x02008fff7000, 0x10007fff7fff]` || HighShadow ||
 // || `[0x00008fff7000, 0x02008fff6fff]` || ShadowGap  ||
 // || `[0x00007fff8000, 0x00008fff6fff]` || LowShadow  ||