Regres: Reduce number of parallel test processes to half system's CPUs

Using all the available CPUs puts too much memory pressure
on the system, and Regres is killed by systemd-oomd as the result.

Change-Id: I45998e8963dca8cedd5f46d192a3fadbe983bcd9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/76748
Presubmit-Ready: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Tested-by: Yuly Novikov <ynovikov@chromium.org>

tests/regres/cmd/regres/main.go

diff --git a/tests/regres/cmd/regres/main.go b/tests/regres/cmd/regres/main.go
index 19ed1f6..be914f3 100644
--- a/tests/regres/cmd/regres/main.go
+++ b/tests/regres/cmd/regres/main.go
@@ -80,7 +80,7 @@
 )
 
 var (
-	numParallelTests = runtime.NumCPU()
+	numParallelTests = (runtime.NumCPU() + 1) / 2
 	llvmVersion      = llvm.Version{Major: 17, Point: 6}
 
 	cacheDir        = flag.String("cache", "cache", "path to the output cache directory")