ldbl-128: Rename 'long double' to '_Float128'

ldbl-128: Rename 'long double' to '_Float128'

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.