[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_fsl_booke.S

@@ -52,9 +52,9 @@
  *   r7 - End of kernel command line string
  *
  */
-	.text
-_GLOBAL(_stext)
-_GLOBAL(_start)
+	.section	.text.head, "ax"
+_ENTRY(_stext);
+_ENTRY(_start);
 	/*
 	 * Reserve a word at a fixed location to store the address
 	 * of abatron_pteptrs