libstdc++: Add missing functions to <cmath> [PR79700]

libstdc++: Add missing functions to <cmath> [PR79700]

This patch adds the -f and -l variants of the C99 <math.h> functions to
<cmath> under namespace std (so std::sqrtf, std::fabsl, etc.) for C++11
and up.

libstdc++-v3/ChangeLog:

	PR libstdc++/79700
	* include/c_global/cmath (acosf, acosl, asinf, asinl, atanf)
	(atanl, atan2f, atan2l, ceilf, ceill, cosf, cosl, coshf, coshl)
	(expf, expl, fabsf, fabsl, floorf, floorl, fmodf, fmodl, frexpf)
	(frexpl, ldexpf, ldexpl, logf, logl, log10f, log10l, modff)
	(modfl, powf, powl, sinf, sinl, sinhf, sinhl, sqrtf, sqrtl, tanf)
	(tanl, tanhf, tanhl): Add using-declarations in namespace std.
	* testsuite/26_numerics/headers/cmath/equivalent_functions.cc:
	New test.
	* testsuite/26_numerics/headers/cmath/functions_std_c++17.cc:
	Add checks for existence of above names.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>