port-amd64/57949: fetestexcept disables all floating-point exception traps as a side effect

[campbell+netbsd%mumble.net@localhost]

>Number:         57949
>Category:       port-amd64
>Synopsis:       fetestexcept disables all floating-point exception traps as a side effect
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Feb 19 21:25:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, 7, 6
>Organization:
The NetBSD Exception
>Environment:
amd64
>Description:
fetestexcept uses fnstenv, which has the side effect of masking all x87 floating-point exception traps.

There's no need to use this; it should just use fnstcw (and stmxcsr).
>How-To-Repeat:
	unsigned enabled0, raised, enabled1;

	feclearexcept(FE_ALL_EXCEPT);
	fedisableexcept(FE_ALL_EXCEPT);

	feenableexcept(FE_ALL_EXCEPT);
	enabled0 = fegetexcept();
	raised = fetestexcept(FE_ALL_EXCEPT);
	enabled1 = fegetexcept();

	printf("enabled0:	0x%x\n", enabled0);
	printf("raised:		0x%x\n", raised);
	printf("enabled1:	0x%x\n", enabled1);

Expected enabled0 and enabled1 to both be 0x3d, but instead enabled0 is 0x3d and enabled1 is 0.
>Fix:
Yes, please!