scons: Increase the minimum version of gcc to 7.

This was bumped up to 7 by policy, but the SConsctruct was still
enforcing the old minimum of version 5.

Change-Id: I91966f95b18451f880af279dae286119f7b282d6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48512
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -351,8 +351,8 @@ else:
           "src/SConscript to support that compiler.")))
 
 if main['GCC']:
-    if compareVersions(main['CXXVERSION'], "5") < 0:
-        error('gcc version 5 or newer required.\n'
+    if compareVersions(main['CXXVERSION'], "7") < 0:
+        error('gcc version 7 or newer required.\n'
               'Installed version:', main['CXXVERSION'])
 
     # Add the appropriate Link-Time Optimization (LTO) flags if `--with-lto` is