[POWERPC] Fix modpost warnings from head*.S on ppc32

We get warnings like the following from the various ppc32 head*.S files:

WARNING: vmlinux.o(.text+0x358): Section mismatch: reference to .init.text:early_init (between 'skpinv' and 'interrupt_base')
WARNING: vmlinux.o(.text+0x380): Section mismatch: reference to .init.text:machine_init (between 'skpinv' and 'interrupt_base')
WARNING: vmlinux.o(.text+0x384): Section mismatch: reference to .init.text:MMU_init (between 'skpinv' and 'interrupt_base')
WARNING: vmlinux.o(.text+0x3aa): Section mismatch: reference to .init.text:start_kernel (between 'skpinv' and 'interrupt_base')
WARNING: vmlinux.o(.text+0x3ae): Section mismatch: reference to .init.text:start_kernel (between 'skpinv' and 'interrupt_base')

Added a .text.head section simliar to what other architectures do since
modpost already excludes this from its warnings.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/head_8xx.S

@@ -38,12 +38,9 @@
 #else
 #define DO_8xx_CPU6(val, reg)
 #endif
-	.text
-	.globl	_stext
-_stext:
-	.text
-	.globl	_start
-_start:
+	.section	.text.head, "ax"
+_ENTRY(_stext);
+_ENTRY(_start);
 
 /* MPC8xx
  * This port was done on an MBX board with an 860.  Right now I only