[MIPS] Discard .exit.text and .exit.data at runtime.

[MIPS] Discard .exit.text and .exit.data at runtime.

While the recent cset ca7402fed2a76cd5a417ac4d375a5539dcb2b6de did improve
things it didn't resolve all the problems.  So bite the bullet and discard
.exit.text and .exit.data at runtime.  Which of course sucks because it
bloats binaries with code that will never ever be used but it's the only
thing that will work reliable as demonstrated by the function sd_major() in
drivers/scsi/sd.c.

Gcc may compile sd_major() using a jump table which it will put into
.rodata.  If it also inlines sd_major's function body into exit_sd() which
gcc > 3.4.x does.  If CONFIG_BLK_DEV_SD has been set to y we would like ld
to discard exit_sd's code at link time.  However sd_major happens to
contain a switch statement which gcc will compile using a jump table in
.rodata on the architectures I checked.  So, when ld later discards
.exit.text only the jump table in .rodata with its stale references to
the discard .exit.text will be left which any no antique ld will honor
with a link error.

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