build: move x64-specific modules into x64 subdirs

First step towards moving x64-specific stuff into arch
subdirectories:

move the submodules in external/ to external/x64/,

rename amd64->x64 in opensolaris,

rename x86_64->x64 in include/api,

adapt build.mk to reference the updated paths,
and improve detection of include dirs and libs in the
external submodules.

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