testsuite: require c99 runtime for trigonometric optimisation tests

testsuite: require c99 runtime for trigonometric optimisation tests

A number of optimisation that simplify trigonometric expressions are only
performed when the compiler knows the target has a C99 libm available.
Since targets like *-elf may not have such a libm, a C99 runtime requirement
is added to these tests.

2020-08-19  Pat Bernardi  <bernardi@adacore.com>

gcc/testsuite/ChangeLog

	* gcc.dg/sinatan-2.c: Add dg-require-effective-target c99_runtime.
	* gcc.dg/sinhovercosh-1.c: Likewise.
	* gcc.dg/tanhbysinh.c: Likewise.