mm: use macros from compiler.h instead of __attribute__((...))

mm: use macros from compiler.h instead of __attribute__((...))

To increase compiler portability there is <linux/compiler.h> which
provides convenience macros for various gcc constructs.  Eg: __weak for
__attribute__((weak)).  I've replaced all instances of gcc attributes with
the right macro in the memory management (/mm) subsystem.

[[email protected]: while-we're-there consistency tweaks]
Signed-off-by: Gideon Israel Dsouza <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>