Export various 128 bit long double functions from libgcc_s.so.1

These were already compiled for some time on aarch64 and riscv, by
including lib/libcompiler_rt/Makefile.inc, but never exported in the
shared library. Since gcc exports these under version GCC_4.6.0, we do
the same.

This review should replace D11482 for now. For e.g. amd64 more work is
still to be done, as compiler-rt does not seem to support 128 bit long
double math for that architecture.

Reviewed by: cem
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28690