Set LCG_hostos correctly and add gcc to Xenv files by default

LbConfiguration/python/LbConfiguration/Platform.py

@@ -128,6 +128,16 @@ def getArchitecture(cmtconfig):
                 architecture = "i686"
     return architecture
 
+def getHostOs(cmtconfig):
+    """ Return the Host os for the given CMTCONFIG.
+    e.g. x86_64-centos7 for x86_64+avx2+fma-centos7-gcc62-opt """
+    arch = getArchitecture(cmtconfig)
+    # Cleaning up the insttruction set to get the base architecture
+    # e.g. x86_64 for  x86_64+avx2+fma
+    arch = arch.split('+')[0]
+    platformType = getPlatformType(cmtconfig)
+    return "_".join([arch, platformType])
+
 def getConfig(architecture, platformtype, compiler, debug=False):
     cmtconfig = None
     if platformtype.startswith("win") :