underflow exceptions

[David Miller]

Hey Joseph, just FYI your test to add underflow exceptions to
libm-test.inc is causing long double testsuite failures on 64-bit
sparc.

Specifically:

testing long double (without inline functions)
Failure: exp2 (-16400) == 0x1p-16400: Exception "Underflow" set
Failure: scalbn (min_value / 4, INT_MAX) == inf: Exception "Underflow" set
Failure: scalbln (min_value / 4, INT_MAX) == inf: Exception "Underflow" set
Failure: scalbln (min_value / 4, LONG_MAX) == inf: Exception "Underflow" set
Failure: scalbln (min_value / 4, 0x88000000) == inf: Exception "Underflow" set

Test suite completed:
  5478 test cases plus 4610 tests for exception flags executed.
  5 errors occurred.

I'll try to dig more deeply but I figured you might be able to tell
offhand whether this is some generic issue on 64-bit cpus or that
it might be prudent to relax the requirements.