Reuse the code for __mpn_extract_long_double to implement
__mpn_extract_float128.
* include/gmp.h: Include bits/floatn.h
(__mpn_extract_float128): Declare when __HAVE_DISTINCT_FLOAT128 is 1.
* stdlib/gmp-impl.h: Also check if alloca is not defined before
including stack-alloc.h. It could have been defined by other header
which not necessarily defines HAVE_ALLOCA.
* sysdeps/ieee754/float128/Makefile: New file.
* sysdeps/ieee754/float128/float1282mpn.c: New file.
* sysdeps/ieee754/float128/float128_private.h: Include gmp.h before
redefining __mpn_extract_long_double to __mpn_extract_float128, then
redefine __mpn_extract_long_double to __mpn_extract_float128.
* sysdeps/ieee754/ldbl-128/ldbl2mpn.c: Replace long double with
_Float128 to allow float128_private.h overrides.
