For microperl, add uconfig64.sh, with values suitable for an LP64 platform.

For microperl, add uconfig64.sh, with values suitable for an LP64 platform.

Add instructions on how to use it to README.micro.

There's rather too much size-specific in perl's configuration to make it viable
to have a single uconfig.sh that will work "properly" on both LP32 and LP64
platforms. It seems best to ship the default as LP32 (which is correct for
Win32 (although I don't know if microperl yet builds on Win32)), and provide
just a config.sh file for LP64, as most (all?) 64 bit *nix platforms are LP64,
and can run the shell commands to generate a uconfig.h from it.

(Win64 is LLP64, so uconfig64.sh isn't going to be right for it, so the
inability of Win64 to generate uconfig.h from uconfig64.sh isn't a problem)