[CI] PR Regressions Slow (~30 Minutes) due to GCC Debug

Nobody has claimed this yet.

The GCC Debug build on the per-PR regressions seem pretty slow. (I've qualitatively felt like this was the case for a while...)

The most egregious issue seems to be that it takes about ~17 minutes to link `handshake-runner`. 🤔

```
2021-09-08T04:58:03.8991892Z [ 65%] Linking CXX executable ../../bin/handshake-runner
...
2021-09-08T05:04:57.3577410Z [100%] Linking CXX executable ../../bin/circt-opt
2021-09-08T05:05:04.3093298Z [100%] Built target circt-opt
2021-09-08T05:15:42.1373633Z [100%] Built target handshake-runner
```

LLVM seems to link this almost instantaneously.

```
2021-09-08T04:52:30.3095758Z [ 64%] Linking CXX executable ../../bin/handshake-runner
2021-09-08T04:52:30.6339054Z [ 64%] Built target handshake-runner
```

A full log is here: https://pipelines.actions.githubusercontent.com/wi15O7TotuPkwX6KT6ySaHTeiMJbgCxjibAprAeUXSFaISA8Od/_apis/pipelines/1/runs/7358/signedlogcontent/19?urlExpires=2021-09-08T05%3A17%3A02.9519975Z&urlSigningMethod=HMACV1&urlSignature=TjtedMpsblZy%2FdplCpO5BGARST96LwoxIVr%2F6kSLF2w%3D

Maybe there's a strategy to speed this up by using `gold` or `lld` for the GCC link. Alternatively, `handshake-runner` may be doing something wonky.

Start by comparing the GCC Debug and LLVM link steps in the supplied full log, focusing on handshake-runner and circt-opt. Investigate whether using gold or lld, or an issue specific to handshake-runner, explains the delay; done means the per-PR GCC Debug build no longer has the roughly 17-minute handshake-runner link.

Written by the indexing model from the issue text.