Use GCC's __attribute__((const)) to reduce code size

__attribute__((const)) is mainly intended for the compiler to optimize
away repeated calls to a function that the compiler knows will return
the same value repeatedly.

By adding __attribute__((const)), the compiler can choose to call the
function just once and cache the return value.  Therefore, this yields
code size reduction.

Here are the reference results by arm-eabi-size for crespo device:

[before]
   text    data     bss     dec     hex filename
 267715   10132   45948  323795   4f0d3

[after]
   text    data     bss     dec     hex filename
 267387   10132   45948  323467   4ef8b

Change-Id: I1d80465c0f88158449702d4dc6398a130eb77195