Restore basic functionality of the C++ process wrapper

[scentini]

Fixes #1530

The process wrapper, among other things, takes care of passing--remap-path-prefix=$PWD= with the correct $PWD value for the Rust compilation actions. The process wrapper is a Rust binary itself, and currently there is nothing to take care of passing --remap-path-prefix when it is being built, which results in nondeterministic process wrapper build. This PR provides a solution for the issue, by building the process wrapper and its dependencies via a minimal C++ process wrapper (which I resurrected from before d9ba374).

We could once again remove the C++ process wrapper once remap-cwd-prefix is stabilized (rust-lang/rust#89434).

[UebelAndre]

Minor cleanup

[UebelAndre]

I did not see #1530 (comment). Sorry for the delay, continuing the thread here. I get consistent results running build->clean->build but I tried doing this in two different checkouts of the same commit and ended up with different results. Which makes me think the remapping flag is actually just not working?

0014fd20: 722f 746d 702f 5f62 617a 656c 5f61 6e64  r/tmp/_bazel_aaa
0014fd30: 7265 6272 6973 636f 2f34 3033 6231 6537  aaaaaaaa/403b1e7
0014fd40: 3037 3666 3861 3865 3932 3739 3334 6630  076f8a8e927934f0
0014fd50: 6137 3039 3135 3930 382f 6578 7465 726e  a70915908/extern
0014fd60: 616c 2f72 7573 745f 6461 7277 696e 5f61  al/rust_darwin_a
0014fd70: 6172 6368 3634 5f5f 6161 7263 6836 342d  arch64__aarch64-
0014fd80: 6170 706c 652d 6461 7277 696e 5f5f 7374  apple-darwin__st
0014fd90: 6162 6c65 5f74 6f6f 6c73 2f6c 6962 2f72  able_tools/lib/r
0014fda0: 7573 746c 6962 2f61 6172 6368 3634 2d61  ustlib/aarch64-a
0014fdb0: 7070 6c65 2d64 6172 7769 6e2f 6c69 622f  pple-darwin/lib/
0014fdc0: 6c69 626c 6962 632d 6334 3639 6264 3437  liblibc-c469bd47
0014fdd0: 6333 3535 3062 3163 2e72 6c69 6228 6c69  c3550b1c.rlib(li
0014fde0: 6263 2d63 3436 3962 6434 3763 3335 3530  bc-c469bd47c3550
0014fdf0: 6231 632e 6c69 6263 2e39 3537 3937 3361  b1c.libc.957973a
0014fe00: 332d 6367 752e 302e 7263 6775 2e6f 2900  3-cgu.0.rcgu.o).
0014fe10: 5f5f 5a4e 346c 6962 6334 756e 6978 3362  __ZN4libc4unix3b
0014fe20: 7364 3561 7070 6c65 3973 6967 696e 666f  sd5apple9siginfo
0014fe30: 5f74 3773 695f 6164 6472 3137 6834 6361  _t7si_addr17h4ca
0014fe40: 3164 3666 6633 3737 6564 6566 6245 0073  1d6ff377edefbE.s
0014fe50: 7263 2f75 6e69 782f 6273 642f 6170 706c  rc/unix/bsd/appl
0014fe60: 652f 6d6f 642e 7273 002f 7275 7374 632f  e/mod.rs./rustc/
0014fe70: 3930 6335 3431 3830 3666 3233 6131 3237  90c541806f23a127
0014fe80: 3030 3264 6535 6234 3033 3862 6537 3331  002de5b4038be731
0014fe90: 6261 3134 3538 6361 2f6c 6962 7261 7279  ba1458ca/library
0014fea0: 2f61 6c6c 6f63 2f73 7263 2f6c 6962 2e72  /alloc/src/lib.r
0014feb0: 732f 402f 0061 6c6c 6f63 2e63 3831 6434  s/@/.alloc.c81d4
0014fec0: 3166 332d 6367 752e 3000 2f70 7269 7661  1f3-cgu.0./priva
0014fed0: 7465 2f76 6172 2f74 6d70 2f5f 6261 7a65  te/var/tmp/_baze
0014fee0: 6c5f 616e 6472 6562 7269 7363 6f2f 3430  l_aaaaaaaaaaa/40
0014fef0: 3362 3165 3730 3736 6638 6138 6539 3237  3b1e7076f8a8e927
0014ff00: 3933 3466 3061 3730 3931 3539 3038 2f65  934f0a70915908/e
0014ff10: 7874 6572 6e61 6c2f 7275 7374 5f64 6172  xternal/rust_dar
0014ff20: 7769 6e5f 6161 7263 6836 345f 5f61 6172  win_aarch64__aar
0014ff30: 6368 3634 2d61 7070 6c65 2d64 6172 7769  ch64-apple-darwi
0014ff40: 6e5f 5f73 7461 626c 655f 746f 6f6c 732f  n__stable_tools/
0014ff50: 6c69 622f 7275 7374 6c69 622f 6161 7263  lib/rustlib/aarc
0014ff60: 6836 342d 6170 706c 652d 6461 7277 696e  h64-apple-darwin
0014ff70: 2f6c 6962 2f6c 6962 616c 6c6f 632d 6164  /lib/liballoc-ad
0014ff80: 3165 6562 3136 6235 3030 3634 3135 2e72  1eeb16b5006415.r
0014ff90: 6c69 6228 616c 6c6f 632d 6164 3165 6562  lib(alloc-ad1eeb
0014ffa0: 3136 6235 3030 3634 3135 2e61 6c6c 6f63  16b5006415.alloc
0014ffb0: 2e63 3831 6434 3166 332d 6367 752e 302e  .c81d41f3-cgu.0.
0014ffc0: 7263 6775 2e6f 2900 5f5f 5a4e 3432 5f24  rcgu.o).__ZN42_$

Note things around var/tmp/_baze. I think this is a pretty big issue so will be trying dedicate any time I can get to it but could definitely use assistance.