While investigating hexops/mach#8 with @slimsag,
we found that zld is forming invalid file paths (absolute paths concatenated together),
which hits the unreachable `OBJECT_NAME_INVALID` case in `openDirAccessMaskW`:
https://github.com/ziglang/zig/blob/0c091feb5ae52caf1ebf885c0de55b3159207001/lib/std/fs.zig#L1522

This is caused by appending `dir` (which is guaranteed to be absolute) to `root`,
an obviously incorrect operation:
https://github.com/ziglang/zig/blob/0c091feb5ae52caf1ebf885c0de55b3159207001/src/link/MachO.zig#L494-L499

Fixes hexops/mach#8

Co-authored-by: Stephen Gutekanst <stephen@hexops.com>
Signed-off-by: Stephen Gutekanst <stephen@hexops.com>
Signed-off-by: Andrew Gutekanst <andrew.gutekanst@gmail.com>