Documentation: raise minimum supported version of binutils to 2.25

Binutils 2.23 was released in 2012. Almost 10 years old.

We already require GCC 5.1, released in 2015.

Bump the binutils version to 2.25, which was released some months
before GCC 5.1.

With this applied, some subsystems can start to clean up code.
Examples:
  arch/arm/Kconfig.assembler
  arch/mips/vdso/Kconfig
  arch/powerpc/Makefile
  arch/x86/Kconfig.assembler

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>