RFC: Add support for CPPFLAGS and clean up CxxFlagsInfo

[allsey87]

While working on #1230, it came to my attention that if CPPFLAGS is included in the env of a configure_make rule, they are also ignored. I think this should be supported like the other xxxFLAGS variables and would like to get feedback on a PR that touches the code as follows:

1. Add CPPFLAGS to _MAKE_FLAGS:
   https://github.com/bazelbuild/rules_foreign_cc/blob/9d5727d5e51bf1be8423dff7996bdb4d847b47e6/foreign_cc/private/make_env_vars.bzl#L79-L88

2. Add cpp to CxxFlagsInfo provider struct:
   https://github.com/bazelbuild/rules_foreign_cc/blob/9d5727d5e51bf1be8423dff7996bdb4d847b47e6/foreign_cc/private/cc_toolchain_util.bzl#L28-L38

I would also like to clean up the names of the variables inside CxxFlagsInfo since they are quite misleading and inconsistent. Linked flags are language in dependent. Would the following be acceptable? I think this provider is just used internally so it would not be a breaking change, right?

CFlagsInfo = provider( 
     doc = "Flags for the C/C++ tools, taken from the toolchain", 
     fields = dict( 
         c = "C compiler flags", 
         cxx = "C++ compiler flags",
         cpp = "C/C++ preprocessor flags",
         linker_shared = "Linker flags when linking shared library", 
         linker_static = "Linker flags when linking static library", 
         linker_executable = "Linker flags when linking executable", 
         asm = "Assembler flags", 
     ), 
 )