ARCH is a user-facing variable and setting it on the command line
clobbers any manipulation attempted in the makefile. Following
the example of the Linux kernel makefiles, use a SRCARCH variable
instead for the internal, potentially fixed-up, architecture name.
This fixes the following error.

make ARCH=x86_64
Makefile:69: *** "The architecture x86_64 isn't supported".  Stop.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>