libdfp: Always link tests with libdfp

This patch adds -Wl,--no-as-needed linker option when building tests to
force linker always use libdfp, even for cases where tests do not
actually uses libdfp symbols (test-numdigits for instance).

This is required because some tests will try to printf decimal floating
point and it requires proper initialization done by libdfp.so constructor.
(specially when building with fortify options).
libdfp: Enable BID for 32bits

This patch add BID support for 32 bits.  64 bits support relies on int
128 bits operations which is supplied by GCC/clang extension
__uint128_t.  Some architectures, however, does not support this
extension (x86 32 bits, for instance).

So on BID 32 bits the mini-gmp implementation from libgmp is used.
Currently it is included direct in libdecnumber/decimal128.c object and
thus used in other parts of libdfp through libdecnumber.a static
library.

The mini-gmp is a multiprecision library that works on limbs of 32 bits.
libdfp: Remove libbid files

With BID support in libdecnumber, there is no need to keep libbid files.
libdecnumber: Fix decDebug64 extension
libdfp: Fix bid-private warning in 32-bits