powerpc64le: add -mno-gnu-attribute to *f128 objects and difftime

In practice, this flag should be applied globally, but it makes a good
sanity check to ensure ibm128 and ieee128 long double files are not
getting mismatched.  _Float128 files use no long double, thus are
always safe to use this option.

Similarly, when investigating the linker complaints, difftime
makes trivial, self contained, usage of long double, so thus it
is also explicitly marked as such.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>