soft-float defines in mips-linux gcc config

[Jay Carlson <nop@nop.com>]

Right now there is a small bug in how mips-linux configures gcc for 
softfloat.
In gcc/config/mips/t-linux, we set up libgcc to include the soft 
floating point code, using the GNU names, like __addsi3.  But because 
mips/linux.h includes mips/ecoff.h, gcc produces calls to the GOFAST 
style names (like dpmul, very namespace-contaminating.)
mips/netbsd.h cleans up after mips/elf.h by doing:

#undef US_SOFTWARE_GOFAST
#undef INIT_SUBTARGET_OPTABS
#define INIT_SUBTARGET_OPTABS

which would fix the problem for mips/linux.h as well.

Jay