[PATCH] frexpl: Support smaller long double.

about https://sourceware.org/pipermail/newlib/2021/018738.html:

> This patch add support for smaller long double, LDBL_MANT_DIG is 24 or 53.

are you sure LDBL_MANT_DIG can be 24 in radix 2? As far as I know, the standard
requires LDBL_DIG >= 10, and a floating-point number with LDBL_DIG decimal
digits should be converted to a long double and back to LDBL_DIG digits
without any modification, which requires LDBL_MANT_DIG >= 34.

Also the standard says in 6.2.5: "the set of values of the
type double is a subset of the set of values of the type long double."

Paul Zimmermann