[sanitizer] Add definitions for Myriad RTEMS platform

Introduce two definitions to be used by the Myriad RTEMS port of the
ASan run-time: SANITIZER_MYRIAD2 for the platform and SANITIZER_RTEMS
for the OS.  We expect to use SANITIZER_MYRIAD2 to guard the portion
of the port corresponding to Myriad's unique memory map, and
SANITIZER_RTEMS for most of the rest.

Differential Revision: https://reviews.llvm.org/D46453

llvm-svn: 331646