second attempt to fix C compiler warnings with -fhpc

second attempt to fix C compiler warnings with -fhpc
The hs_hpc_module() prototype in RtsExternal.h didn't match its usage:
we were passing StgWord-sized parameters but the prototype used C
ints.  I think it accidentally worked because we only ever passed
constants that got promoted.  The constants unfortunately were
sometimes negative, which caused the C compiler to emit warnings.

I suspect PprC.pprHexVal may be wrong to emit negative constants in
the generated C, but I'm not completely sure.  Anyway, it's easy to
fix this in CgHpc, which is what I've done.