Some tweaks for powerpc64-linux

[David Edelsohn]

	We do not need any of the definitions from t-ppcos and very few of
the definitions from t-ppccomm.  In fact, I think all of the definitions
we need from t-ppccomm are in t-rs6000.  (We can easily change t-rs6000 to
use directory "nof" instead of "soft-float", if that seems necessary.)

	Instead of adding t-linux64 to disable earlier definitions, let's
just clean this up.  I think if we define

tmake_file=rs6000/t-rs6000 t-linux rs6000/t-linux64

with t-linux64

# We don't want -fPIC.
CRTSTUFF_T_CFLAGS_S =
TARGET_LIBGCC2_CFLAGS =

# Modify the shared lib version file
SHLIB_MKMAP_OPTS = -v dotsyms=1

that would cover everything.

Thanks, David