cdefs.h: Introduce the __purefunc attribute, which allows us to mark
certain functions as being "pure".
http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
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
