libstdc++: Fix -Wnarrowing in <complex> [PR116991]

When _GLIBCXX_USE_C99_COMPLEX_ARC is undefined we use the generic
__complex_acos function template for _Float32 etc. and that gives a
-Wnarrowing warning:

complex:2043: warning: ISO C++ does not allow converting to '_Float32' from 'long double' with greater conversion rank [-Wnarrowing]

Use a cast to do the conversion so that it doesn't warn.

libstdc++-v3/ChangeLog:

	PR libstdc++/116991
	* include/std/complex (__complex_acos): Cast literal to
	destination type.