[MIPS] Discard .exit.text at runtime.

At times gcc will place bits of __exit functions into .rodata.  If
compiled into the kernle itself we used to discard .exit.text - but
not the bits left in .rodata.  While harmless this did at times result
in a large number of warnings.  So until gcc fixes this, discard
.exit.text at runtime.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>