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/test/toolchains/api_test.py

@@ -155,7 +155,7 @@ def test_gcc_version_check(_run_cmd):
     toolchain = TOOLCHAIN_CLASSES["GCC_ARM"](
         TARGET_MAP["K64F"], notify=notifier)
     toolchain.version_check()
-    assert notifier.messages == []
+    assert len(notifier.messages) == 1
     _run_cmd.return_value = ("""
     arm-none-eabi-gcc (Arch Repository) 8.1.0
     Copyright (C) 2018 Free Software Foundation, Inc.
@@ -165,13 +165,21 @@ def test_gcc_version_check(_run_cmd):
     toolchain.version_check()
     assert len(notifier.messages) == 1
     _run_cmd.return_value = ("""
-    arm-none-eabi-gcc (Arch Repository)
+    arm-none-eabi-gcc (Arch Repository) 9.1.0
     Copyright (C) 2018 Free Software Foundation, Inc.
     This is free software; see the source for copying conditions.  There is NO
     warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
     """, "", 0)
     toolchain.version_check()
     assert len(notifier.messages) == 2
+    _run_cmd.return_value = ("""
+    arm-none-eabi-gcc (Arch Repository)
+    Copyright (C) 2018 Free Software Foundation, Inc.
+    This is free software; see the source for copying conditions.  There is NO
+    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+    """, "", 0)
+    toolchain.version_check()
+    assert len(notifier.messages) == 3
 
 
 @given(fixed_dictionaries({

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,