[ARM] pxa: fix for variables in uncompress.h being discarded

Due to commit:

    5de813b6

  ARM: Eliminate decompressor -Dstatic= PIC hack

The data section will be discarded for the decompressor, thus move the
static variables into BSS section by initializing them at run time.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>