Refactor *cvt functions implementation (1/2)

Changes since v3:

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

Changes since v2:

  - Squashed patches 1-4 and trivial bits of patch 5.
  - Replaced http with https (like the rest of glibc).
  - Updated copyright years.
  - Removed refactoring noise reported by Paul Murphy.

No changes since v1.

-- 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 splitting the implementation per se in one file
(efgcvt-template.c) and the alias definitions in others (e.g. efgcvt.c),
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>