[tlbuild] xz, wget, lz4 updates

[Karl Berry]

There are new versions of xz (https://tukaani.org/xz/xz-5.2.4.tar.xz)
and wget (https://ftp.gnu.org/gnu/wget/wget-1.20.1.tar.gz)
and lz4 (https://github.com/lz4/lz4/archive/v1.8.3.tar.gz)
since the last TL release. 
The updates are not crucial, but if you're up for it, please provide
binaries for your platforms (if necessary, for wget and lz4).

Here is what we are distibuting now:

wget.amd64-freebsd wget.i386-freebsd
wget.amd64-netbsd wget.i386-netbsd
wget.exe
wget.sparc-solaris
wget.x86_64-darwin
wget.x86_64-darwinlegacy
wget.x86_64-solaris wget.i386-solaris

lz4.armhf-linux
lz4.exe
lz4.x86_64-cygwin.exe lz4.i386-cygwin.exe
lz4.x86_64-darwin
lz4.x86_64-linux lz4.i386-linux
lz4.x86_64-linuxmusl
lz4.x86_64-solaris lz4.i386-solaris

Thanks,
Karl