Remove gas and ld support for MIPS ECOFF

[Maciej W. Rozycki]

On Sat, 22 Jun 2013, Richard Sandiford wrote:

> After this patch there's not really any point running the testsuite
> for mips-ecoff.  All that's left are a few binutils tests that fail
> due to lack of assembler support.

 Disabling them altogether may make sense, though is probably not 
essential.  Do we have any test coverage for file format conversion 
though?  At least ELF->ECOFF seems essential, but also some other cases 
involving at least `binary', ECOFF, ELF and SREC would be worthwhile.  
WDYT?

  Maciej