Tools: Restrict toolchains reported by mbed compile -S to official ones

### Description

The `mbed compile -S` command is suposed to indicate what targets
support what toolchains. The command was printing out things that
don't make sense, like `GCC_CR` and things that make sense, but are
not offiially supported yet, like `ARMC6`. This PR fixes all of that.

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change

Author: theotherjimmy

tools/build_api.py

@@ -1072,20 +1072,7 @@ def get_unique_supported_toolchains(release_targets=None):
                       If release_targets is not specified, then it queries all
                       known targets
     """
-    unique_supported_toolchains = []
-
-    if not release_targets:
-        for target in TARGET_NAMES:
-            for toolchain in TARGET_MAP[target].supported_toolchains:
-                if toolchain not in unique_supported_toolchains:
-                    unique_supported_toolchains.append(toolchain)
-    else:
-        for target in release_targets:
-            for toolchain in target[1]:
-                if toolchain not in unique_supported_toolchains:
-                    unique_supported_toolchains.append(toolchain)
-
-    return unique_supported_toolchains
+    return ["ARM", "uARM", "GCC_ARM", "IAR"]
 
 
 def _lowercase_release_version(release_version):