Refactor *cvt functions implementation (2/2)

Changes since v3:

  - Converted into a standalone patch not to be squashed with the
    preceding patch (1/2).
  - Updated commit message.
  - Tested that installed stripped binaries for powerpc64le and x86_64
    remain identical before and after this patch.

Changes since v2:

  - Moved the removal of unused macros to a previous patch

Changes since v1:

  - Adapted after patch v1 19/31 (remove hidden_* uses) was dropped.
  - Fixed unintended removal of cvt* symbols on alpha and s390x
    (now tested with build-many-glibcs.py).

-- 8< --
This patch refactors the *cvt functions implementation in a way that
makes it easier to re-use them for implementing the IEEE long double on
powerpc64le.  By removing the macros that generate the function names
(APPEND combined with FUNC_PREFIX), the new code makes it easier to
define new function names, such as __qecvtieee128.

Tested that installed stripped binaries for powerpc64le and x86_64
remain identical before and after this patch.  Also tested for
powerpc64le, x86_64, as well as with the following build-many-glibcs
targets: alpha-linux-gnu, mips-linux-gnu, powerpc-linux-gnu-soft,
s390x-linux-gnu, and sparc64-linux-gnu.

Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>