kbuild: Fix Makefile.asm-generic for um
kbuild: Fix Makefile.asm-generic for um

Do nothing if arch/$(SRCARCH)/include/asm/Kbuild does not exist, which
is the case of um.

Reported-by: Randy Dunlap <6f29df8e90a57f1ba61ffd63b0468c86bb6d3a99@xenotime.net>
Signed-off-by: Michal Marek <0c835876a532a476f30fdb1f6e963bae6c5a1a04@suse.cz>
Acked-by: Randy Dunlap <6f29df8e90a57f1ba61ffd63b0468c86bb6d3a99@xenotime.net>
Acked-by: Sam Ravnborg <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@ravnborg.org>

TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs