follow change in rev. 1.22 of kern.ldscript and apply same fix for xen
ldscript too, but using just regular PAGE_ALIGN alignment:

"""
Fix a pretty dumb mistake I made in r1.22: the alignment needs to be in the
bss, otherwise the bootloader will use memory before __kernel_end and give
a wrong start pa to the kernel.
"""

this got broke by amd64/locore.S rev. 1.141 which removed an unused variable
which forced the alignment

this fixes ps/bt in ddb to be able to find symbols and hence PR port-xen/53056

(jdolecek)