linux-user/i386: Fix allocation and alignment of fp state

linux-user/i386: Fix allocation and alignment of fp state

For modern cpus, the kernel uses xsave to store all extra
cpu state across the signal handler.  For xsave/xrstor to
work, the pointer must be 64 byte aligned.  Moreover, the
regular part of the signal frame must be 16 byte aligned.

Attempt to mirror the kernel code as much as possible.
Use enum FPStateKind instead of use_xsave() and use_fxsr().

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1648
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>