powerpc: Link VDSOs at 0x0

perf is failing to resolve symbols in the VDSO. A while (1)
gettimeofday() loop shows:

93.99%  [vdso]  [.] 0x00000000000005e0
 3.12%  test    [.] 00000037.plt_call.gettimeofday@@GLIBC_2.18
 2.81%  test    [.] main

The reason for this is that we are linking our VDSO shared libraries
at 1MB, which is a little weird. Even though this is uncommon, Alan
points out that it is valid and we should probably fix perf userspace.

Regardless, I can't see a reason why we are doing this. The code
is all position independent and we never rely on the VDSO ending
up at 1M (and we never place it there on 64bit tasks).

Changing our link address to 0x0 fixes perf VDSO symbol resolution:

73.18%  [vdso]  [.] 0x000000000000060c
12.39%  [vdso]  [.] __kernel_gettimeofday
 3.58%  test    [.] 00000037.plt_call.gettimeofday@@GLIBC_2.18
 2.94%  [vdso]  [.] __kernel_datapage_offset
 2.90%  test    [.] main

We still have some local symbol resolution issues that will be
fixed in a subsequent patch.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/vdso.h

@@ -4,11 +4,11 @@
 #ifdef __KERNEL__
 
 /* Default link addresses for the vDSOs */
-#define VDSO32_LBASE	0x100000
-#define VDSO64_LBASE	0x100000
+#define VDSO32_LBASE	0x0
+#define VDSO64_LBASE	0x0
 
 /* Default map addresses for 32bit vDSO */
-#define VDSO32_MBASE	VDSO32_LBASE
+#define VDSO32_MBASE	0x100000
 
 #define VDSO_VERSION_STRING	LINUX_2.6.15