[fortran,committed] Fix quad-prec NORM2 by creating missing decl for quad-prec BUILT_IN_SQRT

[FX]

BUILT_IN_SQRT is a bit special in the Fortran front-end, because it’s part of our math built-ins which map one-to-one to Fortran intrinsics (and get special treatment for that), but it is only a built-in we call directly in the front-end when emitting code for NORM2.

I didn’t realize this corner case when I first implemented quad-prec in the front-end (now several years ago!). Yet, the fix is simple.

Regtested on x86_64-apple-darwin14. Fixes PR 54656 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54656).
Committed as revision 226929.

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