math: support platforms with limited FP rounding or exception support

For some tests, just claim that fetestexcept() always returns true,
so the rest of the test can be compiled.

For libm-test, provide known bogus values for unsupported rounding
modes, so fesetround() will return failure.

Elsewhere, just add some #ifdefs to avoid code that uses particular
FP exceptions if the exceptions aren't supported.

stdlib/bug-getcontext.c

@@ -9,6 +9,9 @@
 static int
 do_test (void)
 {
+#if FE_ALL_EXCEPT == 0
+  printf("Skipping test; no support for FP exceptions.\n");
+#else
   int except_mask =  FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW | FE_UNDERFLOW;
   int status = feenableexcept (except_mask);
 
@@ -41,6 +44,7 @@ do_test (void)
 
   printf("\nAt end fegetexcept() returned %d, expected: %d.\n",
 	 mask, except_mask);
+#endif
   return 0;
 }