x86: Rename {stop,start}_emulating to fpu_{enable,disable}

[jhb]

Summary

While here, centralize the macros in <x86/fpu.h>.

[markj]

sys/amd64/amd64/fpu.c

803

fpu_enable()?

[jhb]

sys/amd64/amd64/fpu.c

803

I chose clts() here due to the comment above explicitly referencing %cr0.TS, but perhaps fpu_enable() is more correct?

[kib]

sys/amd64/amd64/fpu.c

803

I do not think the specific name there matters. Perhaps the comment should stop referencing lazy mode.

Might be, it is time to remove the workaround as well (I am not too sure about it).

[markj]

sys/amd64/amd64/fpu.c

803

I slightly prefer to be consistent and write fpu_enable().