[patch] testsuite fix for math tests on darwin

[Gabriel Dos Reis]

Andreas Tobler <toa@pop.agri.ch> writes:

| Hi,
| 
| darwin has some C99 math functions in the libmx instead of libm, so we
| should add the -lmx to the tests where we need these functions.
| The attached patch fixes three failures of the libstdc++ suite.
| Unfortunately I did not find the cabsl in libmx.
| 
| Ok for main?

Shouldn't the compiler driver take care of that?  Those functions are
used by libstdc++, so when linking againt libstdc++, the driver should
be passing that additional library too, instead of us setting them in the
testsuite. 

-- Gaby