libgcc_s: export the IEEE-128 long double runtime on powerpc64le

On powerpc64le with IEEE-128 long double, the long-double compiler-runtime
helpers are the *kf* soft-float functions (built from the

libgcc_s: export the IEEE-128 long double runtime on powerpc64le

On powerpc64le with IEEE-128 long double, the long-double compiler-runtime
helpers are the *kf* soft-float functions (built from the tf sources,
renamed via -D in lib/libcompiler_rt/Makefile.inc) plus the complex
multc3/__divtc3. They are compiled into libgcc_s.so by the powerpc64le
SRCF block, but were never added to Symbol.map, so they stayed local and
unexported.

Every other IEEE-128 architecture already exports its scalar long-double
runtime -- aarch64 and riscv list the tf helpers in GCC_4.6.0. powerpc64le
was simply missed.

Because the helpers are unexported, any clang-built shared library that uses
long double leaves them undefined (permitted in a DSO), and linking an
executable against that DSO then fails under lld's default
--no-allow-shlib-undefined. For example science/harminv fails to link its
binary against its own libharminv.so with undefined multc3/divtc3; at
-O0, mulkf3/addkf3/__subkf3/__unordkf2 appear as well.

Export the full runtime, gated on the PowerPC-specific LONG_DOUBLE_IEEE128
predefine so no other architecture is affected: complex multc3/divtc3 in
GCC_4.0.0 (beside the other complex mul*c3), and the 28 scalar *kf*
functions in GCC_7.0.0. Node placement follows glibc/gcc symbol-versioning
history.

Differential Revision: https://reviews.freebsd.org/D58248

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