[tlbuild] build info updates

[Karl Berry]

Hi again. Now that we've been through a round of builds, I'd like to
update the build information for the various platforms, i.e., any
options you're giving to ./Build or however else you're creating these
things. Aside from contextgarden, github, mac, which are online.

So, below is the info I have now in Build/source/README. Johannes,
Simon, Marc, Ken, please let me know about any updates.

Also, Akira, is your build process for w32/64 written down online or in
your distribution, or anywhere?

Thanks.

------------------------------------------------------------
aarch64-linux:
  aarch64 Debian GNU/Linux 10 (buster)
  gcc (Debian 8.3.0-6) 8.3.0
  ./Build --enable-arm-neon=check

armhf-linux:
  ?

i386-netbsd, amd64-netbsd:
  NetBSD/amd64 9.0
  gcc version 7.4.0 (nb3 20190319)
    TL_MAKE=gmake CC=gcc CXX=g++ \
    CFLAGS=-D_NETBSD_SOURCE \
    CXXFLAGS='-D_NETBSD_SOURCE -std=c++11' \
    LDFLAGS='-L/usr/X11R7/lib -Wl,-rpath,/usr/X11R7/lib' \
  ./Build --enable-xindy CLISP=/usr/local/bin/clisp
  Except that we take Nelson's asy build.

i386-cygwin, x86_64-cygwin: gcc-4.8.2, cygwin-1.7.29.
  ./Build (with --enable-xindy for i386).