LDBL_EQ_DBL Multilib Issues

[Joel Sherrill]

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?
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