arm: Comment out unused labels in simple_idct_arm

When building for iOS in thumb mode, gas-preprocessor.pl doesn't
mark unused labels as thumb functions (as it does for other
local labels, where it can figure out that they are functions
due to being referenced in branch instructions). This leads to
linker warnings for some of those local labels, such as:

ld: warning: ARM function not 4-byte aligned: __a_evaluation from
libavcodec/libavcodec.a(simple_idct_arm.o)

Therefore, comment them out since they don't have any function.
They do still have a value in documenting key points in the
assembly source though.

Signed-off-by: Martin Storsjö <martin@martin.st>