Emit the unsupported option error until the Clang's library integration support for 128-bit long double is available for AIX.
It's not that AIX doesn't support 128-bit long double. The xlc compiler has -qldbl128, which links in different libraries for 128-bit. The XL compilers use -lc128 and -lC128 so there are binary compatibility concerns that need to be investigated.
I suggest this comment be updated accordingly.
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