Auto-detect 64-bit on Solaris/sparc64

[solardiz]

We auto-detect Solaris 10 and 11 sparc64 systems as 32-bit and produce a working 32-bit build, but gmake -f Makefile.legacy solaris-sparc64-gcc produces a working 64-bit build, which runs faster - and by the way it also needs -m64 on linking there.

[solardiz]

$ uname -mrsp
SunOS 5.10 sun4u sparc

This says just sparc, not sparc64. So we'd have to check something like uname -mrs for regexp ^SunOS 5\.1. sun4u. This feels a bit hackish.

Also, we wouldn't want to override a user's explicit choice of sparc (32-bit) for $host_cpu.

And then if they choose to keep 32-bit as the default build target, installed third-party libraries could happen to be 32-bit only. On the test systems now I'm maybe just lucky to have 64-bit OpenSSL as well.

Overall, this might not be such a good idea.

Maybe instead we could document how to request a 64-bit build, maybe have configure suggest re-invoking it that way.

./configure --build=sparc64-solaris