[PATCH, PPC/Darwin] Fix long double symbol exports.

[Mike Stump]

On Dec 6, 2018, at 11:52 AM, Iain Sandoe <iain@sandoe.co.uk> wrote:
> 
> During 8.x, the rs6000 target-specific mangling was reorganised which uncovered
> a long-standing bug in Darwin’s mangling for ‘IBM’ long double.  Now the symbols
> are correctly mangled, and we end up with a bunch of test link fails.
> 
> This patch adds the necessary subset of the Linux long double exports to Darwin’s
> export table.
> 
> I have tested this on a few bootstrap/regtest cycles on powerpc-darwin9, and on the
> power7 linux system.
> 
> For the record, I’ve noted the library versions from the Linux side, although Darwin
> does not version symbols in this way.
> 
> OK for trunk and 8.x?

Don't know if the libstdc++ want to review this or they want me to...  I'm fine with it.