Revert "Add the exception in Mbed OS build tool for Arm Compiler 5 toolchain."

For backwards compatibility reasons, as the Mbed tools must be able to
build any online-compiler-supported version of Mbed, revert back to
emitting a warning when ARMC5 is used. This will enable ARMC5 to
continue working in the online compiler for Mbed OS 5 and other previous
versions. ARMC5 remains unsupported in Mbed 6.

This reverts commit 03d57b7ba2.

tools/build_api.py

@@ -47,6 +47,7 @@ from .notifier.mock import MockNotifier
 from .targets import TARGET_NAMES, TARGET_MAP, CORE_ARCH, Target
 from .libraries import Library
 from .toolchains import TOOLCHAIN_CLASSES, TOOLCHAIN_PATHS
+from .toolchains.arm import ARMC5_MIGRATION_WARNING
 from .toolchains.arm import UARM_TOOLCHAIN_WARNING
 from .toolchains.mbed_toolchain import should_replace_small_c_lib
 from .config import Config
@@ -244,11 +245,9 @@ def find_valid_toolchain(target, toolchain):
             ).format(toolchain_name, search_path)
         else:            
             if toolchain_name == "ARMC5":
-                raise NotSupportedException(
-                    "Arm Compiler 5 is no longer supported, please upgrade to Arm Compiler 6."
-                )                
+                end_warnings.append(ARMC5_MIGRATION_WARNING)
             if (
-                toolchain_name in ["uARM", "ARMC6"] 
+                toolchain_name in ["uARM", "ARMC5", "ARMC6"] 
                 and "uARM" in {toolchain_name, target.default_toolchain}
             ):
                 end_warnings.append(UARM_TOOLCHAIN_WARNING)

tools/toolchains/arm.py

@@ -32,6 +32,14 @@ from tools.toolchains.mbed_toolchain import (
 from tools.utils import mkdir, NotSupportedException, run_cmd
 from tools.resources import FileRef
 
+ARMC5_MIGRATION_WARNING = (
+    "Warning: We noticed that you are using Arm Compiler 5. "
+    "We are deprecating the use of Arm Compiler 5. "
+    "Please upgrade your environment to Arm Compiler 6 "
+    "which is free to use with Mbed OS. For more information, "
+    "please visit https://os.mbed.com/docs/mbed-os/latest/tools/index.html"
+)
+
 UARM_TOOLCHAIN_WARNING = (
     "Warning: We noticed that you are using uARM Toolchain either via --toolchain command line or default_toolchain option. "
     "We are deprecating the use of the uARM Toolchain. "