Add CMake long double cross-compile defaults (#3683)

HDF5 performs a couple of checks at build time to see if long double
values can be converted correctly (IBM's Power architecture uses a
special format for long doubles). These checks were performed using
TRY_RUN, which is a problem when cross-compiling.

These checks now use default values appropriate for most non-Power
systems when cross-compiling. The cache values can be pre-set if
necessary, which will preempt both the TRY_RUN and the default.

Affected values:
    H5_LDOUBLE_TO_LONG_SPECIAL      (default no)
    H5_LONG_TO_LDOUBLE_SPECIAL      (default no)
    H5_LDOUBLE_TO_LLONG_ACCURATE    (default yes)
    H5_LLONG_TO_LDOUBLE_CORRECT     (default yes)
    H5_DISABLE_SOME_LDOUBLE_CONV    (default no)

Fixes GitHub #3585

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