Take a step toward complete BB domination (JuliaLang/julia#33125)

This provides a number of benefits:

* Significantly reduces the amount of work in win-extras, which I've
always thought was an annoying wart in our build system.

* Provides a consistent 7z executable on all platforms, which can be
picked up by Pkg/BinaryProvider for consistent
unzipping/untarring/ungzipping on all platforms. Note that on Windows,
7z is a repackaged binary from sourceforge and is the "full" 7z, whereas
on all other platforms it is a p7zip build of the standalone 7za
executable with all relevant patches applied. There are some format
capability differences, but for the important things it should be fine.

(cherry picked from commit 16225e6)

LICENSE.md

@@ -76,11 +76,10 @@ Julia's build process uses the following external tools:
 - [PATCHELF](https://nixos.org/patchelf.html)
 - [OBJCONV](https://www.agner.org/optimize/#objconv)
 
-Julia bundles the following external programs and libraries on some platforms:
+Julia bundles the following external programs and libraries:
 
 - [7-Zip](https://www.7-zip.org/license.txt)
 - [ZLIB](https://zlib.net/zlib_license.html)
-- [LIBEXPAT](http://expat.cvs.sourceforge.net/viewvc/expat/expat/README)
 
 On some platforms, distributions of Julia contain SSL certificate authority certificates,
 released under the [Mozilla Public License](https://en.wikipedia.org/wiki/Mozilla_Public_License).