fix ppc-vxworks not to clobber explicit options for E500

[Olivier Hainque]

Hello Joseph,

Joseph S. Myers wrote:
> >    #define SUBSUBTARGET_OVERRIDE_OPTIONS           \

> Although such a check on rs6000_explicit_options seems correct, it would 
> also seem in line with the intention of this code in vxworks.h for the 
> default to be 1 for -mcpu=8540 and 2 for -mcpu=8548 - which would however 
> require proper separate CPU entries for 8540 and 8548 (including making 
> the scheduler in 8540.md apply for both CPUs where it presently checks for 
> "ppc8540").

 Hmm, I agree that having proper handling of the 8548 would be a nice
 improvement. However, it seems to me that the check not to override explicit
 requests is useful either way, so the two look pretty orthogonal IMO.

 Olivier