Now that sparc64 is using gcc-3.x we can disallow gcc-2.91, etc.

Documentation/Changes already says 2.95.3, which is working fine for me.

With this change, we no longer require that per-cpu data definitions be
initialised.  That was a workaround for a bug in older gccs.  So remove the
build infrastructure which was checking for that.

Also, mention that nfs-utils-1.0.3 is required.  It isn't required yet, but
will be once we enable larger dev_t: there is an interface for exportfs which
passes dev_t's into the kernel which breaks with larger dev_t.  That
interface is old, deprecated and is not used in nfs-utils-1.0.3.