Use volatile global counters in test-tgmath.c.

If build with -Os on s390x, the test-tgmath fails with:
float functions not called often enough (-10000)

Within compile_testf(), the counter (count_float) is saved
before the complex functions are called.
Afterwards the saved counter differs to the current-counter.

But the tests with the complex functions do not increment count_float!
Instead count_float is saved to a register before calling totalorder
and totalordermag which both increment count_float.
The compiler is allowed to do that as totalorderf and totalordermagf
is declared with __attribute__ ((__const__)) in math/bits/mathcalls.h.

Thus this patch adjusts the global counters to be volatile.
Then count_float is saved after totalordermag.

ChangeLog:

	* math/test-tgmath.c (count_double, count_float,
	count_ldouble, count_cdouble, count_cfloat,
	count_cldouble): Use volatile int.