powerpc/ptrace: Make user_mode() common to PPC32 and PPC64

Today we have:

	#ifdef __powerpc64__
	#define user_mode(regs) ((((regs)->msr) >> MSR_PR_LG) & 0x1)
	#else
	#define user_mode(regs) (((regs)->msr & MSR_PR) != 0)
	#endif

With ppc64_defconfig, we get:

	if (!user_mode(regs))
    14b4:	e9 3e 01 08 	ld      r9,264(r30)
    14b8:	71 29 40 00 	andi.   r9,r9,16384
    14bc:	41 82 07 a4 	beq     1c60 <.emulate_instruction+0x7d0>

If taking the ppc32 definition of user_mode(), the exact same code
is generated for ppc64_defconfig.

So, only keep one version of user_mode(), preferably the one not
using MSR_PR_LG which should be kept internal to reg.h.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/000a28c51808bbd802b505af42d2cb316c2be7d3.1629216000.git.christophe.leroy@csgroup.eu