OMPILE) prefix missing from size invocation

Otherwise, cross compilation may fail with error messages like:

...
size: arch/arm/boot/compressed/../../../../vmlinux: File format is ambiguous
size: Matching formats: elf32-littlearm elf32-littlearm-symbian elf32-littlearm-vxworks
  LD      arch/arm/boot/compressed/vmlinux
  arm-angstrom-linux-uclibcgnueabi-ld:--defsym _kernel_bss_size=: syntax error

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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