I have observed a bug on 7.4.0 whereby __mulkc3 calls are
swapped with __multc3 depending on ABI selection. For the
sake of being overly cautious, build all _Float128 files
with ibm128 to workaround these compilers. This has been
noted in GCC BZ 84914, and will not be fixed for GCC 7.
Likewise, non-math files built with _Float128 are assumed
to have ibm long double. Explicilty preserve this
assumption.
Finally, add some bootstrapping code to avoid applying
these options until IEEE long double is enabled as they
require GCC 7 and above.
Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
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