lib: Add shared copies of some GCC library routines

Many ports (m32r, microblaze, mips, parisc, score, and sparc) use
functionally identical copies of various GCC library routine files,
which came up as we were submitting the RISC-V port (which also uses
some of these).

This patch adds a new copy of these library routine files, which are
functionally identical to the various other copies.  These are
availiable via Kconfig as CONFIG_GENERIC_$ROUTINE, which currently isn't
used anywhere.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>