string.h: add __attribute__ ((pure)) to string functions

cdefs.h: Introduce the __purefunc attribute, which allows us to mark
certain functions as being "pure".

  Many functions have no effects except the return value and their
  return value depends only on the parameters and/or global variables.
  Such a function can be subject to common subexpression elimination
  and loop optimization just as an arithmetic operator would be.

string.h: Mark many commently used string functions as "pure", to
allow for additional compiler optimizations.

Change-Id: I42961f90f822b6dbcbc3fd72cdbe774a7adc8785
2 files changed