Refactor *cvt functions implementation (2/2)

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 all build-many-glibcs
targets remain identical before and after this patch.  Also tested for
powerpc64le and x86_64.

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