[patch,target] Fix ppc-darwin issue with long double intrinsics (PR25477)

[FX Coudert]

Thanks to Andrew's code, Mike's and Geoff's comments in the PR, help from Uros, Paolo and Jack, and Dominique's machine for testing, here is a patch for fixing this PR. It has three independent parts, joined together because I regtested them together:

FX

--
François-Xavier Coudert
http://www.homepages.ucl.ac.uk/~uccafco/

1. the target part, in gcc/config/darwin* and gcc/config/rs6000, that takes care of setting correct assembler names for the builtins, if needed 2. the Fortran front-end part, bordering on obvious, that makes cpow{f,,l} builtins instead of simply considering them library functions (otherwise, they don't benefit from the patch above) 3. the Fortran testsuite part, splitting testing of ERF/ERFC from large_real_kind_2.F90 into its separate test case, which still fails on Darwin at -O0 because Apple's PowerPC erfl() and ercl() are plain useless. The patch was bootstrapped on powerpc-apple-darwin9.2.0 with C and Fortran, and regtested with both -m32 and -m64 for these same languages. OK for trunk? (I need a Darwin maintainer approval for the target stuff, and a Fortran maintainer for Fortran parts.)

darwin-longdouble-4.ChangeLog
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darwin-longdouble-4.diff
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