powerpc/vdso: Fix DOTSYM for 32-bit LE VDSO

Skirmisher reported on IRC that the 32-bit LE VDSO was hanging. This
turned out to be due to a branch to self in eg. __kernel_gettimeofday.
Looking at the disassembly with objdump -dR shows why:

  00000528 <__kernel_gettimeofday>:
   528:   f0 ff 21 94     stwu    r1,-16(r1)
   52c:   a6 02 08 7c     mflr    r0
   530:   f0 ff 21 94     stwu    r1,-16(r1)
   534:   14 00 01 90     stw     r0,20(r1)
   538:   05 00 9f 42     bcl     20,4*cr7+so,53c <__kernel_gettimeofday+0x14>
   53c:   a6 02 a8 7c     mflr    r5
   540:   ff ff a5 3c     addis   r5,r5,-1
   544:   c4 fa a5 38     addi    r5,r5,-1340
   548:   f0 00 a5 38     addi    r5,r5,240
   54c:   01 00 00 48     bl      54c <__kernel_gettimeofday+0x24>
                          54c: R_PPC_REL24        .__c_kernel_gettimeofday

Because we don't process relocations for the VDSO, this branch remains
a branch from 0x54c to 0x54c.

With the preceding patch to prohibit R_PPC_REL24 relocations, we
instead get a build failure:

  0000054c R_PPC_REL24       .__c_kernel_gettimeofday
  00000598 R_PPC_REL24       .__c_kernel_clock_gettime
  000005e4 R_PPC_REL24       .__c_kernel_clock_gettime64
  00000630 R_PPC_REL24       .__c_kernel_clock_getres
  0000067c R_PPC_REL24       .__c_kernel_time
  arch/powerpc/kernel/vdso32/vdso32.so.dbg: dynamic relocations are not supported

The root cause is that we're branching to `.__c_kernel_gettimeofday`.
But this is 32-bit LE code, which doesn't use function descriptors, so
there are no dot symbols.

The reason we're trying to branch to a dot symbol is because we're
using the DOTSYM macro, but the ifdefs we use to define the DOTSYM
macro do not currently work for 32-bit LE.

So like previous commits we need to differentiate if the current
compilation unit is 64-bit, rather than the kernel as a whole. ie.
switch from CONFIG_PPC64 to __powerpc64__.

With that fixed 32-bit LE code gets the empty version of DOTSYM, which
just resolves to the original symbol name, leading to a direct branch
and no relocations:

  000003f8 <__kernel_gettimeofday>:
   3f8:   f0 ff 21 94     stwu    r1,-16(r1)
   3fc:   a6 02 08 7c     mflr    r0
   400:   f0 ff 21 94     stwu    r1,-16(r1)
   404:   14 00 01 90     stw     r0,20(r1)
   408:   05 00 9f 42     bcl     20,4*cr7+so,40c <__kernel_gettimeofday+0x14>
   40c:   a6 02 a8 7c     mflr    r5
   410:   ff ff a5 3c     addis   r5,r5,-1
   414:   f4 fb a5 38     addi    r5,r5,-1036
   418:   f0 00 a5 38     addi    r5,r5,240
   41c:   85 06 00 48     bl      aa0 <__c_kernel_gettimeofday>

Fixes: ab037dd87a ("powerpc/vdso: Switch VDSO to generic C implementation.")
Reported-by: "Will Springer <skirmisher@protonmail.com>"
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201218111619.1206391-3-mpe@ellerman.id.au

arch/powerpc/include/asm/ppc_asm.h

@@ -180,7 +180,12 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
 #define VCPU_GPR(n)	__VCPU_GPR(__REG_##n)
 
 #ifdef __KERNEL__
-#ifdef CONFIG_PPC64
+
+/*
+ * We use __powerpc64__ here because we want the compat VDSO to use the 32-bit
+ * version below in the else case of the ifdef.
+ */
+#ifdef __powerpc64__
 
 #define STACKFRAMESIZE 256
 #define __STK_REG(i)   (112 + ((i)-14)*8)