x86: Initialise FPU regs during thread creation for eager FPU sharing

When "eager FPU sharing" mode is enabled, FPU registers must be
initialised at the time of thread creation because the floating-point
context is always active and no further FPU initialisation is performed
later.

Note that, in case of the "lazy FPU sharing" mode, floating-point
context is inactive by default and the FPU is initialised when the
first floating-point instruction is executed.

Refer to the issue #44902 for more details.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>