[sanitizer] Add definitions for Myriad RTEMS platform

[waltl]

Summary

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.

[kcc]

LGTM

[vitalybuka]

One of your changes is probably responsible for http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/6690

lib/sanitizer_common/sanitizer_platform.h

 #endif
 
 #if defined(__Fuchsia__)
 # define SANITIZER_FUCHSIA 1
 #else
 # define SANITIZER_FUCHSIA 0
 #endif
 
+#if defined(__rtems__)
+# define SANITIZER_RTEMS 1
+#else
+# define SANITIZER_RTEMS 0
+#endif
+
 #define SANITIZER_POSIX \
   (SANITIZER_FREEBSD || SANITIZER_LINUX || SANITIZER_MAC || \
     SANITIZER_NETBSD || SANITIZER_OPENBSD || SANITIZER_SOLARIS)
 
 #if __LP64__ || defined(_WIN64)
 #  define SANITIZER_WORDSIZE 64
 #else
 #  define SANITIZER_WORDSIZE 32

 #endif
 
 #if SANITIZER_SOLARIS && SANITIZER_WORDSIZE == 32
 # define SANITIZER_SOLARIS32 1
 #else
 # define SANITIZER_SOLARIS32 0
 #endif
 
+#if defined(__myriad2__)
+# define SANITIZER_MYRIAD2 1
+#else
+# define SANITIZER_MYRIAD2 0
+#endif
+
 // By default we allow to use SizeClassAllocator64 on 64-bit platform.
 // But in some cases (e.g. AArch64's 39-bit address space) SizeClassAllocator64
 // does not work well and we need to fallback to SizeClassAllocator32.
 // For such platforms build this code with -DSANITIZER_CAN_USE_ALLOCATOR64=0 or
 // change the definition of SANITIZER_CAN_USE_ALLOCATOR64 here.
 #ifndef SANITIZER_CAN_USE_ALLOCATOR64
 # if (SANITIZER_ANDROID && defined(__aarch64__)) || SANITIZER_FUCHSIA
 #  define SANITIZER_CAN_USE_ALLOCATOR64 1