* stage2: put decls in different MachO sections

Use `getDeclVAddrWithReloc` when targeting MachO backend rather than
`getDeclVAddr` - this fn returns a zero vaddr and instead creates a
relocation on the linker side which will get automatically updated
whenever the target decl is moved in memory. This fn also records
a rebase of the target pointer so that its value is correctly slid
in presence of ASLR.

This commit enables `zig test` on x86_64-macos.

* stage2: fix output section selection for type,val pairs