x86/fpu: Create a typedef for the x87/SSE area inside "struct xsave_struct"

x86/fpu: Create a typedef for the x87/SSE area inside "struct xsave_struct"

Making the union non-anonymous would cause a lot of headaches, because a lot of
code relies on it being so, but it's possible to make a typedef of the anonymous
union so all callsites currently relying on typeof() can stop doing so directly.

This commit creates a `fpusse_t` typedef to the anonymous union at the head of
the XSAVE area and uses it instead of typeof().

No functional change.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Jan Beulich <jbeulich@suse.com>