fpu: save each process's x87/SSE registers; keep the kernel off them

Nothing ever saved the FPU and SSE registers on a context switch.
musl's memcpy, the compiler's struct copies and any arithmetic on
doubles keep live values in %xmm, so two processes sharing a CPU wrote
over each other's: two shells counting in a loop on one processor took
a NULL+8 page fault within seconds, at a different rip every time. It
reproduces on kernels from before this week too -- it had always been
there, and every crash nobody could explain was a candidate.

sched() now fxsaves the process leaving the CPU and scheduler()
fxrstors the one arriving. A new process starts from the ABI's state
(FCW 0x37F, MXCSR 0x1F80), fork copies the parent's live registers,
exec resets them, and a signal handler's are put back by rt_sigreturn
so the interrupted code finds what it had.

The kernel was free to use %xmm too -- GCC vectorised struct copies
and memset into 153 instructions of it -- so an interrupt could
clobber the registers of the program it interrupted with no switch at
all. Kernel, lwIP and uACPI objects are built -mgeneral-regs-only now
and the image has none; cmd/ keeps SSE.

Checked under QEMU: three concurrent loops three times over, on 1 CPU
and on 4, no faults; before, every run faulted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/exec.c

@@ -389,6 +389,10 @@ do_exec(char *path, struct argsrc *av, struct argsrc *ev)
   console_takeover();
 
   switchuvm(myproc());
+  // A new program starts with the FPU as the ABI promises it, not with the
+  // old one's registers.
+  memmove(myproc()->fpu, fpu_default, sizeof(myproc()->fpu));
+  fxrstor(myproc()->fpu);
   freevm(oldpgdir);
   kfree((char*)scratch);
   kfree(bounce);