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

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

libc/include/sys/cdefs.h

@@ -306,9 +306,11 @@
 #if __GNUC_PREREQ__(2, 96)
 #define __noreturn    __attribute__((__noreturn__))
 #define __mallocfunc  __attribute__((malloc))
+#define __purefunc    __attribute__((pure))
 #else
 #define __noreturn
 #define __mallocfunc
+#define __purefunc
 #endif
 
 /*