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]

Documentation/process/changes.rst

@@ -31,7 +31,7 @@ you probably needn't concern yourself with isdn4k-utils.
 ====================== ===============  ========================================
 GNU C                  4.6              gcc --version
 GNU make               3.81             make --version
-binutils               2.20             ld -v
+binutils               2.21             ld -v
 flex                   2.5.35           flex --version
 bison                  2.0              bison --version
 util-linux             2.10o            fdformat --version
@@ -77,9 +77,7 @@ You will need GNU make 3.81 or later to build the kernel.
 Binutils
 --------
 
-The build system has, as of 4.13, switched to using thin archives (`ar T`)
-rather than incremental linking (`ld -r`) for built-in.a intermediate steps.
-This requires binutils 2.20 or newer.
+Binutils 2.21 or newer is needed to build the kernel.
 
 pkg-config
 ----------