cheriot-baremetal: depend on compiler emitting -D__CHERIOT_BAREMETAL__

The CHERIoT compiler automatically emits a #define for __CHERIOT_BAREMETAL__
when run with -mabi=cheriot-baremetal. Replace old (manual) usage of
_CHERIOT_BAREMETAL_.

Bypass-Presubmit-Reason: camera b0rked breaks Nexus test; missing bazelversion breaks sencha test

Change-Id: If78d0ec974353232691165fc8fd553c1d62ff5ba

sw/device/lib/testing/test_rom/BUILD

diff --git a/sw/device/lib/testing/test_rom/BUILD b/sw/device/lib/testing/test_rom/BUILD
index 6474063..e57fb2e 100644
--- a/sw/device/lib/testing/test_rom/BUILD
+++ b/sw/device/lib/testing/test_rom/BUILD
@@ -172,7 +172,7 @@
 ) for target in TARGETS]
 
 # Only build for CHERIoT targets. Requires that bazel be invoked with
-# --config=cheriot-baremetal --copt=-D_CHERIOT_BAREMETAL_=1
+# --config=cheriot-baremetal
 # so that all dependencies use those options.
 opentitan_rom_binary(
     name = "test_rom_no_otp_cheri",

sw/device/lib/testing/test_rom/cheriot-baremetal.h

diff --git a/sw/device/lib/testing/test_rom/cheriot-baremetal.h b/sw/device/lib/testing/test_rom/cheriot-baremetal.h
index 38bed19..7ad3a51 100644
--- a/sw/device/lib/testing/test_rom/cheriot-baremetal.h
+++ b/sw/device/lib/testing/test_rom/cheriot-baremetal.h
@@ -17,7 +17,7 @@
 #define SW_DEVICE_LIB_TESTING_TEST_ROM_CHERIOT_BAREMETAL_H_
 
 // XXX just make this a noop?
-#ifndef _CHERIOT_BAREMETAL_
+#ifndef __CHERIOT_BAREMETAL__
 #error "This file is only useful when compiling for baremetal"
 #endif