[patch,libgfortran] Fix binary128 ERFC_SCALED

[FX]

This patch fixes libgfortran’s binary128 [aka real(kind=16)] variant of 
ERFC_SCALED. The original code, which I had lifted from netlib, gives only 18 
significant decimal digits, which is not enough for binary128 (33 decimal 
digits).

I thus implemented a new variant for binary128. For arguments < 12, it simply 
calls erfcq() then multiplies by expq(x*x). For larger arguments, it uses a 
power expansion in 1/x. The new implementation provides answers within to 2 ulp 
of the correct value.

Regtested on x86_64-apple-darwin13, comes with a testcase. OK to commit?
FX

erfc_scaled.ChangeLog
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erfc_scaled.diff
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