[v3] Fix / clean-up config vs crosses (5/n): remove [GLIBCXX_CHECK_BUILTIN_MATH_SUPPORT]

[Paolo Carlini]

Hi,

in this case, I'm trying to remove something that seems very badly 
obsolete to me. First, of the various builtins checked only 4 results 
are actually used, in libmath/mathconf.h. In turn, the latter uses (in 
order to possibly replace normal library functions in math.h?!?) would 
be certainly unsafe these days, because 
[GLIBCXX_CHECK_BUILTIN_MATH_DECL_AND_LINKAGE_1] tries to compile and 
link calls with a compile-time constant argument, which are completely 
optimized out.

Thus, I'm going to wait until tomorrow in case I'm missing something, 
then will apply.

Paolo.

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