The linux-headers/asm-arm/unistd.h file has been split in three
sub-files, copy them along.  However, building them requires
setting ARCH rather than SRCARCH.

SRCARCH defaults to $(ARCH) anyway; to avoid future occurrence of
the same problem use ARCH for all architectures where SRCARCH=ARCH.
Currently these are all except x86, sparc, sh and tile.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170221122920.16245-2-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>