Support LOCAL_MODULE_RELATIVE_PATH

Most users of LOCAL_MODULE_PATH are setting a subdirectory of the
normal install path, for example to install HALs into system/lib/hw.
This is problematic for multiarch builds, where the install location
depends on the arch.  Allow modules to specify LOCAL_MODULE_RELATIVE_PATH.
HALs will generally use:
LOCAL_MODULE_RELATIVE_PATH := hw

Change-Id: I4e4ceec61d026bbe74ba604554c06104bde42e5e