use the fp target attribute with fpu_save()/fpu_load()

otherwise clang 18 errors when inline assembly uses floating point
registers with nofp in -march

ok kettenis@

Rewrite the kernel FPU handling code. The new code saves the FPU state
in cpu_switch() instead of at the kernel edge and gets rid of the FPU
state tracking in struct cpu_info and struct pcb. This f

Rewrite the kernel FPU handling code. The new code saves the FPU state
in cpu_switch() instead of at the kernel edge and gets rid of the FPU
state tracking in struct cpu_info and struct pcb. This fixes the random
crashes seen with SMP kernels on Apple M1.

ok patrick@

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