macho: reduce heap allocations

[kubkon]

Memory reduced to 1444.9MB from 2757.87MB according to Tracy.

Linker performance virtually unchanged:

$ hyperfine ./zld ./zld_old
Benchmark 1: ./zld
  Time (mean ± σ):      4.351 s ±  0.027 s    [User: 3.547 s, System: 2.467 s]
  Range (min … max):    4.308 s …  4.401 s    10 runs

Benchmark 2: ./zld_old
  Time (mean ± σ):      4.345 s ±  0.020 s    [User: 3.844 s, System: 2.106 s]
  Range (min … max):    4.319 s …  4.381 s    10 runs

Summary
  ./zld_old ran
    1.00 ± 0.01 times faster than ./zld