_LINUX: used to detect a target build is a linux kernel/application.
__linux__: used to detect a build is on a linux hosts.

Thus we can see: if a linux kernel build is performed on environments other
than linux hosts, __linux__ may not be defined by the compiler and _LINUX
cannot cover linux kernel resident ACPICA files, as it's only defined in
<linux/acpi.h> and hence only allows non ACPICA kernel files to correctly
include aclinux.h.
As a conclusion, we don't actually support such build.

This patch adds -D_LINUX for ACPICA files so that kernel builds on any
hosts can use unified _LINUX as a linux kernel target indication to
correctly include aclinux.h.
Tested-by: Al Stone <ahs3@redhat.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>