Significantly speed up assembling of sysroots

Significantly speed up assembling of sysroots

By avoiding some redundant rustc calls and stripping debuginfo for
wrappers. ./y.rs build --sysroot none now runs 44% faster.

Benchmark 1: ./y_before.bin build --sysroot none
  Time (mean ± σ):      2.200 s ±  0.038 s    [User: 2.140 s, System: 0.653 s]
  Range (min … max):    2.171 s …  2.303 s    10 runs

Benchmark 2: ./y_after.bin build --sysroot none
  Time (mean ± σ):      1.528 s ±  0.020 s    [User: 1.388 s, System: 0.490 s]
  Range (min … max):    1.508 s …  1.580 s    10 runs

Summary
  './y_after.bin build --sysroot none' ran
    1.44 ± 0.03 times faster than './y_before.bin build --sysroot none'