Tools: check for GCC 8, rather than 6

Planning to update to supporting GCC 8 as found in GNU Tools for Arm
Embedded Processors 8-2018q4-major.

Newer GCC in particular supports ARMv8-M security extensions, and
link-time optimisation - two areas of interest.

Should be no code changes required, but need to change the warning in
the toolchain script.

Author: kjbracey

tools/toolchains/gcc.py

@@ -33,7 +33,7 @@ class GCC(mbedToolchain):
     STD_LIB_NAME = "lib%s.a"
     DIAGNOSTIC_PATTERN = re.compile('((?P<file>[^:]+):(?P<line>\d+):)(?P<col>\d+):? (?P<severity>warning|[eE]rror|fatal error): (?P<message>.+)')
 
-    GCC_RANGE = (LooseVersion("6.0.0"), LooseVersion("7.0.0"))
+    GCC_RANGE = (LooseVersion("8.0.0"), LooseVersion("9.0.0"))
     GCC_VERSION_RE = re.compile(b"\d+\.\d+\.\d+")
 
     def __init__(self, target,  notify=None, macros=None, build_profile=None,