speed up canned small-icu build

[srl295]

Version: HEAD
Platform: !win (at this point)
Subsystem: deps/icu

When using canned small-icu (the default for configure && make), AND on little endian systems, we can avoid building some things we're currently building.

This isn't a great speedup, but it does:

• reduce the number of files to compile from about 2,130 to 1,901 - so about 11% of the files reduced
• On my test machine, a compile went from 27m17s to 25m55s - so about 5%
• Reduce a few of the system headers needed (maybe) in some scenarios such as Failed to compile from source for armv7l (busybox) on  #7173

(details: in this scenario, only genccode needs to be compiled on the ICU side, and it doesn't need the i18n or io libraries).

[srl295]

let me know if there's any interest in this. It speeds up the default for people doing configure so could be worthwhile.

[mscdex]

Anything to speed up build time is good IMHO, even if it's not a speedup on all platforms.

[srl295]

@mscdex thanks.

• nothing windows specific here- just didn't port the change yet

[jasnell]

What's the status on this?

[srl295]

@jasnell probably nothing that wouldn't still build, just needs testing.

[jasnell]

ping @srl295

[srl295]

close this in favor of #19214 (we won't want to do both!)