Fixing issue where GCC fails to report compile errors when non-verbose

The output of the GCC compiler is such that the toolchain regex sometimes
got hung up on the ':' charcter being printed in front of the drive letter when
running on Windows. This PR changes the matching logic to be more flexible
by using 'search' to check the entire string for a match, not just the
beginning of the string.

tools/toolchains/gcc.py

@@ -138,7 +138,7 @@ class GCC(mbedToolchain):
         # The warning/error notification is multiline
         msg = None
         for line in output.splitlines():
-            match = GCC.DIAGNOSTIC_PATTERN.match(line)
+            match = GCC.DIAGNOSTIC_PATTERN.search(line)
             if match is not None:
                 if msg is not None:
                     self.cc_info(msg)