port-alpha/60479: alpha fails to save/restore fpu registers on signal delivery

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60479
>Category:       port-alpha
>Synopsis:       alpha fails to save/restore fpu registers on signal delivery
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-alpha-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jul 22 20:55:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        
>Organization:
The AlphaBSD Fpundation, Inc.
>Environment:
>Description:

	FPU registers are not saved and restored on signal delivery,
	according to tests/kernel/t_signal_and_fpu:

https://releng.netbsd.org/b5reports/alpha/2026/2026.07.21.22.53.03/test.html#kernel_t_signal_and_fpu_double

	The code appears to be there but it doesn't seem to be working:

https://nxr.netbsd.org/xref/src/sys/arch/alpha/alpha/machdep.c?r=1.385#1868

>How-To-Repeat:

	cd /usr/tests/kernel && atf-run t_signal_and_fpu

	(The test result in the float/double/ldouble tests is the
	number of iterations before the signal is delivered and the FPU
	registers appear to get trashed.)

>Fix:

	Yes, please!