isl: add support for Android libmesa_isl static library

isl: add support for Android libmesa_isl static library

isl library is needed to build i965, libmesa_isl static library is added
to fix related Android building errors.

Any attempt to build libmesa_genxml as phony package module failed to deliver
gen{7,75,8,9}_pack.h generated headers, needed for libmesa_isl_gen{7,75,8,9}

Due to constraints in Android Build System, libmesa_genxml is built as static,
at least one source is needed, so dummy.c is autogenerated for this scope,
libmesa_genxml dependency is declared using LOCAL_WHOLE_STATIC_LIBRARIES,
to avoid building errors due to missing genxml/gen{7,75,8,9}_pack.h headers.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>