LDBL_EQ_DBL Multilib Issues

Hi

I wondered when we reorganized LDBL_EQ_DBL logic if there wouldn't be a
multilib edge case. Chris encountered it.

Attached is a simple test program based on the autoconf probe in
configure.ac that can be preprocessed with different CPU/ABI flags. The
following shows that the default on m68k is not the same as for the 5282.

$ m68k-rtems6-gcc -O2 -E ldbl.c | less

$ m68k-rtems6-gcc -O2 -E -mcpu=5282 ldbl.c | less

Turns out the default flags end up with LDBL_EQ_DBL isn't defined but it
should be for -mcpu=5282 builds.

I don't know any mechanism to add header file variants in newlib by
multilib, so I we need to consider moving this to a dynamic check.

Any thoughts on where to move this?

/* guts of test from newlib/configure.ac */
#include <float.h>
#if DBL_MANT_DIG == LDBL_MANT_DIG  &&  LDBL_MIN_EXP == DBL_MIN_EXP  && \
    LDBL_MAX_EXP == DBL_MAX_EXP
  #define _LDBL_EQ_DBL
 #else
  #error "LDBL != DBL"
#endif

/* just to get values */
int sDBL_MANT_DIG = DBL_MANT_DIG;
int sLDBL_MANT_DIG = LDBL_MANT_DIG;
int sLDBL_MIN_EXP = LDBL_MIN_EXP;
int sDBL_MIN_EXP = DBL_MIN_EXP;
int sLDBL_MAX_EXP = LDBL_MAX_EXP;
int sDBL_MAX_EXP = DBL_MAX_EXP;

const char *ldbl_eq_dbl =
#ifdef LDBL_EQ_DBL
  "defined";
#else
  "undefined";
#endif