Add compat shim for CHERI annotations.

Change-Id: I57322cc82de9c077e456297b12768a199b75ea92

sw/device/lib/base/macros.h

diff --git a/sw/device/lib/base/macros.h b/sw/device/lib/base/macros.h
index 718115e..b06cac6 100644
--- a/sw/device/lib/base/macros.h
+++ b/sw/device/lib/base/macros.h
@@ -423,4 +423,12 @@
  */
 #define OT_ALIGN_MEM(x) (uint32_t)(4 + (((uintptr_t)(x)-1) & ~3))
 
+/**
+ * CHERI platform compat annotation shims. CHERI-enabled toolchains have
+ * __cheri* macros defined; for all other environments we elide their use.
+ */
+#ifndef __cheri_libcall
+#define __cheri_libcall
+#endif
+
 #endif  // OPENTITAN_SW_DEVICE_LIB_BASE_MACROS_H_