modula-2: Remove uses of scalb*() and significand*() [PR107631]

modula-2: Remove uses of scalb*() and significand*() [PR107631]

The scalb*() functions are obsolete in Posix from 2004 and removed in
2008.

The significand*() functions are glibc-only and considered there to be
obsolescent (not supported for types such as _Float128).

We can remove them from Modula-2 since they are not required for ISO
support, but we need to provide an implementation of significand* for
the "fraction()" functions.

	PR modula2/107631

gcc/m2/ChangeLog:

	* gm2-gcc/m2builtins.cc: Remove scalb, scalbf, scalbl,
	significand, significandf, significandl.
	* gm2-libs/Builtins.def (significand): Likewise.
	* gm2-libs/Builtins.mod: Likewise.
	* target-independent/Builtins.texi: Likewise.
	* gm2-libs-iso/LowLong.mod: Implement fraction with scalbn*() and
	ilogb*().
	* gm2-libs-iso/LowReal.mod: Likewise.
	* gm2-libs-iso/LowShort.mod: Likewise.