lib/60539: XSAVE changes break ucontext userspace API

["nia%sent.at@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60539
>Category:       lib
>Synopsis:       XSAVE changes break ucontext userspace API
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 04 20:00:00 +0000 2026
>Originator:     nia
>Release:        NetBSD 11.99.7
>Organization:
The NetBSD __foundation_t
>Environment:
NetBSD siphon 11.99.7 NetBSD 11.99.7 (SIPHON) #0: Sun Aug  2 22:56:47 CEST 2026  nia@siphon:/encrypt/obj/sys/arch/amd64/compile/SIPHON amd64
>Description:
The compiler error:
WasmSignalHandlers.cpp:177:24: error: invalid cast from type
'__fpregset_t' to type 'fxsave*'

The source code in question:
# define XMM_sig(p,i) (((struct fxsave64*)(p)->uc_mcontext.__fpregs)->fx_xmm[i])

This is from an old version of the Mozilla codebase:
https://repo.palemoon.org/MoonchildProductions/UXP/src/commit/4d5b1cc5d8597958e9bbed46b60752395d9d0532/js/src/wasm/WasmSignalHandlers.cpp#L177
>How-To-Repeat:

>Fix: