kbuild: Raise the minimum required binutils version to 2.21

kbuild: Raise the minimum required binutils version to 2.21

It helps to use some new instructions directly in assembly code.

Suggested-by: Andi Kleen <[email protected]>
Signed-off-by: Chang S. Bae <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Acked-by: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Ravi Shankar <[email protected]>
Cc: Linux Torvalds <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]