Unify and bump some of the NMBCLUSTERS defines. Some archs had it set to

Unify and bump some of the NMBCLUSTERS defines. Some archs had it set to
4MB which is far too low especially when the platform is able to run MP.
New limits are, amd64 = 256M; arm64, mips64, sparc64 = 64M; alpha, arm,
hppa, i386, powerpc = 32M; m88k, sh = 8M
Still rather conservative numbers but much better than before. At least
some hangs of arm64 build boxes was caused by this.
OK kettenis@, visa@