Rust 1.58.0 unfortunately doesn't provide facilities to control symbol
visibility/presence, but we care about controlling the list of
symbols exported from libruby-static.a and libruby.so.

This commit uses `ld -r` to make a single object out of rustc's
staticlib output, libyjit.a. This moves libyjit.a out of MAINLIBS and adds
libyjit.o into COMMONOBJS, which obviates the code for merging libyjit.a
into libruby-static.a. The odd appearance of libyjit.a in SOLIBS is also
gone.

To filter out symbols we do not want to export on ELF platforms, we use
objcopy after the partial link. On darwin, we supply a symbol list to
the linker which takes care of hiding unprefixed symbols.

[Bug #19255]

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>