MFC r320056: arm: set appropriate section flags for .init_pagetable

The arm kernel linker scripts place the .init_pagetable section in .bss,
but .init_pagetable had no section flags set, and so did not match the
expected flags for .bss.

GNU ld silently ignores this case, but lld reports an error:

ld: error: incompatible section flags for .bss

locore.o:(.init_pagetable): 0x0
output section .bss: 0x3

PR: 220055
Sponsored by: The FreeBSD Foundation