MIPS: Move & rename fpu_emulator_{save,restore}_context

MIPS: Move & rename fpu_emulator_{save,restore}_context

These functions aren't directly related to the FPU emulator at all, they
simply copy between a thread's saved context & a sigcontext. Thus move
them to the appropriate signal files & rename them accordingly. This
makes it clearer that the functions don't require the FPU emulator in
any way.

Signed-off-by: Paul Burton <[email protected]>
Reviewed-by: Qais Yousef <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/6422/
Signed-off-by: Ralf Baechle <[email protected]>