powerpc/ftrace: No need to read LR from stack in _mcount()

All functions calling _mcount do it exactly the same way, with the
following sequence of instructions:

	c07de788:       7c 08 02 a6     mflr    r0
	c07de78c:       90 01 00 04     stw     r0,4(r1)
	c07de790:       4b 84 13 65     bl      c001faf4 <_mcount>

Allthough LR is pushed on stack, it is still in r0 while entering
_mcount().

Function arguments are in r3-r10, so r11 and r12 are still available
at that point.

Do like PPC64 and use r12 to move LR into CTR, so that r0 is preserved
and doesn't need to be restored from the stack.

While at it, bring back the EXPORT_SYMBOL at the end of _mcount.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/24a3ba7db388537c44a038026f926d885372e6d3.1635423081.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/trace/ftrace_32.S

@@ -14,16 +14,16 @@ _GLOBAL(mcount)
 _GLOBAL(_mcount)
 	/*
 	 * It is required that _mcount on PPC32 must preserve the
-	 * link register. But we have r0 to play with. We use r0
+	 * link register. But we have r12 to play with. We use r12
 	 * to push the return address back to the caller of mcount
 	 * into the ctr register, restore the link register and
 	 * then jump back using the ctr register.
 	 */
-	mflr	r0
-	mtctr	r0
-	lwz	r0, 4(r1)
+	mflr	r12
+	mtctr	r12
 	mtlr	r0
 	bctr
+EXPORT_SYMBOL(_mcount)
 
 _GLOBAL(ftrace_caller)
 	MCOUNT_SAVE_FRAME
@@ -43,7 +43,6 @@ _GLOBAL(ftrace_graph_stub)
 	/* old link register ends up in ctr reg */
 	bctr
 
-EXPORT_SYMBOL(_mcount)
 
 _GLOBAL(ftrace_stub)
 	blr