Symbols files under $(OUT)/symbols/bionic/

We need symbol files for /bionic/* paths.

New property "mountsource" is added to specify the "real" module that
the bionic mountpoint module is a mountpoint for. The real module
provides path to the unstripped elf file, which is installed to the
symbols/bionic/* path.

Bug: 123985838
Test: m libc.mountpoint libm.mountpoint libdl.mountpoint
linker.mountpoint
$(OUT)/symbols/bionic/bin/linker, bionic/lib[64]/lib{c|dl|m}.so exist

Change-Id: I43f074f0076b576f214fe92a98689a413efd3daa