OSL: Cleanup AcpiOsMapMemory()/AcpiOsUnmapMemory() stubs.

In order to make acpidump a portable applicatiion it will need to link
osunixxf.c/oswinxf.c, which will create 2 instances of AcpiOsMapMemory() and
AcpiOsUnmapMemory(). This patch introduces ACPI_USE_NATIVE_MEMORY_MAPPING as a
mechanism to exclude using the the generic verisons of these functions in
acpidump.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>