Revert "tests: work around bad segment in timer_irq test."

Now that the ELF headers are in a different section we can revert this
workaround.

This reverts commit 4e30a450f05152d60f05b7211e31662809620a9d.
tests: don't have ELF headers merged into .text segment.

Otherwise we ended up where _start came first at 0x20000000 and then the
segment was expanded downards to fit in the ELF headers into memory that
didn't actually exist.