0.2.4: linker.ld: Use explict program headers

This maps the file headers into readonly data, while preventing any
other sections from being generated. It also makes the file 8% smaller.

We also replace the marker symbols for {begin|end}_of_{text|data} with
symbols denoting the begining/end of the file itself.

Finally, we make an explict DISCARD section (based off of EDK2's
GccBase.lds to prevent any additional data from entering our binary).

Signed-off-by: Joe Richey <joerichey@google.com>