math: Add inf support on gen-auto-libm-tests.c

math: Add inf support on gen-auto-libm-tests.c

For some correctly rounded inputs where infinity might generate
a number (like atanf), comparing to a pre-defined constant does not
yield the expected result in all rounding modes.

The most straightforward way to handle it would be to get the expected
result from mpfr, where it handles all the rounding modes.