powerpc/signal32: Transform save_user_regs() and save_tm_user_regs() in 'unsafe' version

Change those two functions to be used within a user access block.

For that, change save_general_regs() to and unsafe_save_general_regs(),
then replace all user accesses by unsafe_ versions.

This series leads to a reduction from 2.55s to 1.73s of
the system CPU time with the following microbench app
on an mpc832x with KUAP (approx 32%)

Without KUAP, the difference is in the noise.

	void sigusr1(int sig) { }

	int main(int argc, char **argv)
	{
		int i = 100000;

		signal(SIGUSR1, sigusr1);
		for (;i--;)
		    raise(SIGUSR1);
		exit(0);
	}

An additional 0.10s reduction is achieved by removing
CONFIG_PPC_FPU, as the mpc832x has no FPU.

A bit less spectacular on an 8xx as KUAP is less heavy, prior to
the series (with KUAP) it ran in 8.10 ms. Once applies the removal
of FPU regs handling, we get 7.05s. With the full series, we get 6.9s.
If artificially re-activating FPU regs handling with the full series,
we get 7.6s.

So for the 8xx, the removal of the FPU regs copy is what makes the
difference, but the rework of handle_signal also have a benefit.

Same as above, without KUAP the difference is in the noise.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Fixup typo in SPE handling]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c7b37b385ccf9666066452e58f018a86573f83e8.1597770847.git.christophe.leroy@csgroup.eu