build: provide simple starting point for crosscompilation

this implements simple target detection borrowing from the
idea previously in Makefile.
Adds basic recognition of passed envs ARCH, CROSS_PREFIX,
along with CC, LD, CXX, HOST_CXX.
HOST_CXX is necessary because the compiler is used to
produce and run binaries at build time.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>