Move some float128 symbol version definitions.

With support for _Float128 functions on platforms where that type has
the same ABI as long double, as well as on platforms where it is
ABI-distinct, those functions will need to be exported from glibc's
shared libraries at appropriate symbol versions in each case.

This patch avoids duplication of lists of symbols to export by moving
the symbols other than __* to math/Versions and stdlib/Versions.
There, they are conditional on <float128-abi.h> defining
FLOAT128_VERSION and a default version of that header is added that
does not define that macro.  Enabling the float128 function aliases
will then include adding a sysdeps/ieee754/ldbl-128/float128-abi.h
that defines FLOAT128_VERSION to GLIBC_2.27.  Symbols __* remain in
sysdeps/ieee754/float128/Versions; those symbols should be present
only once per floating-point format, not once per type.

Note that if any platforms currently lacking support for a type with
binary128 format get glibc support for such a type in future (whether
only as _Float128, or also as a new long double format), and new libm
functions (present for all types) have been added by then, additional
macros will be needed to allow such functions to get a version of the
form "GLIBC_2.28 if the platform had _Float128 support by then, or the
later version at which that platform had _Float128 support added".
This is not however a preexisting condition, but would have applied
equally to the existing support for _Float128 as an ABI-distinct
type.  New all-type libm functions should just be added to the
appropriate symbol version (currently GLIBC_2.27) for all types, with
such special-case handling for _Float128 versions (and _Float64x as
well in future) waiting until someone actually wants to add support
for _Float128 to an existing platform after a release in which that
platform and a post-2.26 libm function had support but that platform
lacked _Float128 support.

Tested with build-many-glibcs.py that installed stripped shared
libraries are unchanged by this patch.  Also tested in conjunction
with the remaining changes to enable float128 aliases.

	* sysdeps/generic/float128-abi.h: New file.
	* sysdeps/ieee754/float128/Versions (FLOAT128_VERSION): Move
	non-__prefixed symbols to ....
	* math/Versions: ... here.  Include <float128-abi.h>.
	* stdlib/Versions ... and here.  Include <float128-abi.h>

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