[asan] Add missing #include of sanitizer_platform.h

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.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D118296