macho: fix perf bug in DWARF parsing

[kubkon]

Fixes #138

Drops link time in #138 from approx 22.70s to approx 6.28s as measured with time utility, and brings peak RSS down from approx 16GB to 2.4GB as measured with tracy. In both cases I measured the following command:

$ xcrun clang++ -o foo objs/* -fuse-ld=/Users/kubkon/dev/zld/zig-out/bin/ld64.zld

The performance when running via zig itself is similar:

$ time ~/dev/zig/build/stage3/bin/zig c++ -o foo objs/*
________________________________________________________
Executed in    6.55 secs    fish           external
   usr time    5.74 secs    0.12 millis    5.74 secs
   sys time    2.77 secs    2.00 millis    2.77 secs

[kubkon]

Matching changes in Zig upstream can be found in ziglang/zig@ed7073c Before I hit the merge button on both, I wanna ponder adding some more coverage for debug info for macho, however the changes can be used to build the compiler and test against the sources in #138.