Fix two bugs with the kernel image compression:
1) reset the bss section of the compressed vmlinux
2) clear the high half of the registers for 64 bit early enough
   for the decompression step
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

Add the "bzImage" compile target and the necessary code  to generate
compressed kernel images. The old style uncompressed "image" target
is preserved, a simple make will build them both.
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>