Need for GMP 5.0.5

[Torbjorn Granlund]

I believe we will need a GMP 5.0.5 release.

(1) GMP 5.0.4 recognises AMD 11h processors as 10h (aka K10), then
    making use of the LZCNT instruction.  Unfortunately, 11h processors
    while later than 10h processors, don't implement all K10
    instructions, and also don't raise an illegal instruction trap.  The
    impact is the worst possible, crashes all over the place.

(2) GMP 5.0.4 fails to recognise newer Intel sandybridge processors due
    to a typo of "coreisrb" for "coreisbr" in GMP's config.guess.  The
    impact is not too bad, just a build failure.

Any other urgent changes needed?

-- 
Torbjörn