[tlbuild] update to xetex - please rebuild if possible

[Jonathan Kew]

My apologies to all builders, but xetex in the TL source tree has just  
been updated to 0.999.3 (r.9515, changes to web2c/texmfmem.h and web2c/ 
xetexdir/xetex.ch).

The reason for this is that dumped format files contained TFM font  
information in an endian-specific form, so .fmt files could not always  
be shared across different architectures. If possible, it would be  
good to rebuild and install the new xetex binary for all platforms (no  
other binaries are affected).

If it's not possible for some people to rebuild, the previous version  
is functionally identical except for the potential non-portability of  
formats, so it isn't a critical issue.

JK