name of cc in the renamed rumprun toolchain

[Antti Kantee]

Hi,

At least the NetBSD cross-toolchain distinguishes between gcc and clang 
in the cc binary name, i.e. arch--netbsd-gcc or arch--netbsd-clang. 
Should we do the same?  Obviously, it would always be -gcc for now, 
before some kind soul comes up with clang support, but at least we 
should be set later on.

If so, should we keep -cc as an alias for -gcc, so as to simplify 
instructions to not always have to contain "if you want to use gcc, set 
CC to -gcc, if you want to use clang, set CC to -clang"?

  - antti