Change support vector to Windows 8+

Doing this makes binaries tinier, since we don't need to have all the
extra code for supporting a 32-bit address space. It also benefits us
because we're able to use WIN32 futexes, which makes locking simpler.

b69f3d2488 is what officially ended our
Windows 7 support. This change is merely a formalization. You can use
old versions of Cosmo now and forevermore if you need Windows 7 since
our repository is hermetic and vendors all its dependencies.

Won't fix #617

tool/scripts/man2txt

@@ -1,5 +1,9 @@
 #!/bin/sh
+
+ASCII2UTF8=$(command -v ascii2utf8.com 2>/dev/null) || {
+  ASCII2UTF8=$(ls -1 o/*/tool/viz/ascii2utf8.com | head -n1)
+}
+
 for x; do
-  nroff -mandoc -rLL=80n -rLT=80n -Tutf8 <"$x" |
-    o//tool/viz/ascii2utf8.com
+  nroff -mandoc -rLL=80n -rLT=80n -Tutf8 <"$x" | $ASCII2UTF8
 done