powerpc/32: Don't use a struct based type for pte_t

Long time ago we had a config item called STRICT_MM_TYPECHECKS
to build the kernel with pte_t defined as a structure in order
to perform additional build checks or build it with pte_t
defined as a simple type in order to get simpler generated code.

Commit 670eea92 ("powerpc/mm: Always use STRICT_MM_TYPECHECKS")
made the struct based definition the only one, considering that the
generated code was similar in both cases.

That's right on ppc64 because the ABI is such that the content of a
struct having a single simple type element is passed as register,
but on ppc32 such a structure is passed via the stack like any
structure.

Simple test function:

	pte_t test(pte_t pte)
	{
		return pte;
	}

Before this patch we get

	c00108ec <test>:
	c00108ec:	81 24 00 00 	lwz     r9,0(r4)
	c00108f0:	91 23 00 00 	stw     r9,0(r3)
	c00108f4:	4e 80 00 20 	blr

So, for PPC32, restore the simple type behaviour we got before
commit 670eea92

, but instead of adding a config option to
activate type check, do it when __CHECKER__ is set so that type
checking is performed by 'sparse' and provides feedback like:

	arch/powerpc/mm/pgtable.c:466:16: warning: incorrect type in return expression (different base types)
	arch/powerpc/mm/pgtable.c:466:16:    expected unsigned long
	arch/powerpc/mm/pgtable.c:466:16:    got struct pte_t [usertype] x

With this patch we now get

	c0010890 <test>:
	c0010890:	4e 80 00 20 	blr

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Define STRICT_MM_TYPECHECKS rather than repeating the condition]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c904599f33aaf6bb7ee2836a9ff8368509e0d78d.1631887042.git.christophe.leroy@csgroup.eu