Documentation: Drop or replace remaining mentions of IA64

Drop or update mentions of IA64, as appropriate.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
diff --git a/Makefile b/Makefile
index ceb23ee..f13b8db 100644
--- a/Makefile
+++ b/Makefile
@@ -378,7 +378,7 @@
 # When performing cross compilation for other architectures ARCH shall be set
 # to the target architecture. (See arch/* for the possibilities).
 # ARCH can be set during invocation of make:
-# make ARCH=ia64
+# make ARCH=arm64
 # Another way is to have ARCH set in the environment.
 # The default ARCH is the host where make is executed.
 
@@ -386,7 +386,7 @@
 # during compilation. Only gcc and related bin-utils executables
 # are prefixed with $(CROSS_COMPILE).
 # CROSS_COMPILE can be set on the command line
-# make CROSS_COMPILE=ia64-linux-
+# make CROSS_COMPILE=aarch64-linux-gnu-
 # Alternatively CROSS_COMPILE can be set in the environment.
 # Default value for CROSS_COMPILE is not to prefix executables
 # Note: Some architectures assign CROSS_COMPILE in their arch/*/Makefile