Remove unused floating-point configuration from gmp-impl.h.

Remove unused floating-point configuration from gmp-impl.h.

This patch removes the IEEE_DOUBLE_BIG_ENDIAN and
IEEE_DOUBLE_MIXED_ENDIAN macros from gmp-impl.h and gmp-mparam.h, and
the ieee_double_extract union from gmp-impl.h.  The macros were used
only in defining the union, which was used nowhere in glibc.  As GMP's
gmp-impl.h is over 5000 lines, the file in glibc is so far from the
GMP version that it doesn't seem to make sense to keep things there
that are not relevant in glibc.  (I expect there is plenty more in the
header after this patch that is also not relevant in glibc and can be
cleaned up later.)

Tested with build-many-glibcs.py that installed stripped shared
libraries are unchanged by this patch.