Add a layer of macro indirection for long double files
which need to be built using another typename. Likewise,
add the L(num) macro used in a later patch to override
real constants.
These macros are only defined through the ldbl-128
math_ldbl.h header, thereby implicitly restricting
these macros to machines which back long double
with an IEEE binary128 format.
Likewise, appropriate changes are made for the few
files which indirectly include such ldbl-128 files.
These changes produce identical binaries for s390x,
aarch64, and ppc64.
A number of files share identical code for the
mul_split function.
This moves the duplicated function mul_split into its
own header, and refactors the fma usage into a single
selection macro. Likewise, mul_split when used by a
long double implementation is renamed mul_splitl for
clarity.
