Run compiler specs in release mode

[straight-shoota]

Building the compiler (or compiler specs in this case) in release mode has a hefty upfront cost, but it improves execution speed of the compiler a lot. The extra overhead is not worth it if you only use the compiler a couple of times, but when you use it a lot, it makes sense to build in release mode.

The compiler specs are pretty huge and thus use the compiler a lot. So I figured, why not try build compiler_spec in release mode to speed up the execution? Compiler specs are a huge portion of CI job runtime, so maybe we can speed things up a bit.

So I did that in this branch and compared the runtimes with previous runs on master.
The duration of spec runs varies quite a bit, actually. So it must be taken with a grain of salt. I did only one CI run in release mode yet.
But there's a clear tendency: Each individual CI job which runs compiler_spec finished more quickly in release mode than in previous runs. The speed up is typically between 3-10 minutes.

As an example, this is the comparison of the x86_64-gnu-test jobs in Linux CI:

This branch	#13079
35:59	45:13
42:57	49:32
35:26	46:01
43:40	55:58
45:27	48:42
35:45	45:16
34:49	46:18
40:05	45:27

The total sum of durations dropeed from 6:22:27 to 5:14:08.

So running specs in release mode seems to bring quite a noticable performance improvement.