tools/elfutils: split upstreamable parts of portability patch

tools/elfutils: split upstreamable parts of portability patch

The addition of LT_INIT as well as the adjustment of
the BUILD_STATIC and addition of the BUILD_SHARED conditionals
and their usage to block building of shared objects
and adjust the variables for building static libraries
is potentially upstream-friendly.

The use of a manifest file to keep a list
of the objects in each library instead of calling ar
is also potentially upstream-friendly.

Separate these changes from the macOS-specific hacks.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15690
Signed-off-by: Robert Marko <robimarko@gmail.com>