bit32: new module for lua5.1 compatibility.

* specs/posix_bit32_spec.yaml: Specify behaviours of band, bnot
and bor.
* specs/specs.mk (specl_SPECS): Add posix_bit32_spec.yaml.
* ext/posix/bit32.c: New module implementing band, bnot and bor.
* ext/posix/posix.c: #include bit32.c.
* build-aux/config.ld.in (file): Add ext/posix/bit32.c.
* local.mk (EXTRA_ext_posix_posix_la_SOURCES): Likewise.
(posix_submodules): Likewise.
(ext_posix_bit32_la_LDFLAGS): Set module LDFLAGS.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>