x86: Explicitly declare adress of .phys.bss
x86: Explicitly declare adress of .phys.bss

Sometimes when linking the linker relocates the .phys.bss section to
the 0 physical address (seemingly ignoring the `.` directive). To work
around this we calculate an explicit address based on the size of the
.phys section and set that as the segment address. This *should* be
completely equivalent to just using `.`, but ld seems to disagree

cmr/seL4,cmr/seL4,cmr/seL4