CONFIG: combine wmake rules for KNL architecture as a compile option

- instead of

      WM_COMPILER=GccKNL WM_COMPILE_OPTION=Opt

      -> linux64GccKNLDPInt32Opt

  now specify

      WM_COMPILER=Gcc  WM_COMPILE_OPTION=OptKNL

      -> linux64GccDPInt32OptKNL

This makes it easier (and more obvious) for adding different tweaks
without needing to generate too many files.
Eg,

    cd wmake/rules/linux64Gcc

    cp cOpt   cOptBdw
    cp c++Opt c++OptBdw

    edit these two files and then use WM_COMPILE_OPTION=OptBdw

CONFIG: provide some default c/c++ flags in General compiler rules

- can make is easier when deriving new compile options, and ensures
  that '-02' is enabled as an initial default.

wmake/rules/linuxARM64Gcc/c

@@ -1,9 +1,7 @@
-SUFFIXES += .c
+include $(GENERAL_RULES)/Gcc/c
 
 cc          = gcc
 
-cWARN       = -Wall
-
 include $(DEFAULT_RULES)/c$(WM_COMPILE_OPTION)
 
 cFLAGS      = $(GFLAGS) $(cWARN) $(cOPT) $(cDBUG) $(LIB_HEADER_DIRS) -fPIC