[sw] track cheriot-baremetal #define

The toolchain now emits __CHERIOT_BAREMETAL__ instead of
_CHERIOT_BAREMETAL_.

Change-Id: I719b248afca49ab826822259b747b328263b2394

sw/device/lib/base/memory.h

diff --git a/sw/device/lib/base/memory.h b/sw/device/lib/base/memory.h
index c4a1682..d895dd5 100644
--- a/sw/device/lib/base/memory.h
+++ b/sw/device/lib/base/memory.h
@@ -18,7 +18,7 @@
 #include <stdint.h>
 
 // NB: --mabi=cheriot-baremetal generates direct C calls & unmangled symbols
-#ifdef _CHERIOT_BAREMETAL_
+#ifdef __CHERIOT_BAREMETAL__
 #undef  __cheri_libcall
 #define __cheri_libcall
 #endif