[SystemZ] Mangle long double as __float128

In r239421, the mangling of long double on PowerPC Linux targets
was changed to use "g" instead of "e".  This same change also needs
to be done for SystemZ (all targets, since we support only Linux
on SystemZ anyway).

This is because an old ABI variant set "long double" to a 64-bit
type equivalent to "double", and the "e" mangling code is still
used to refer to that old ABI for compatibility reasons.

llvm-svn: 239822