bench-perf: use -N1 for single-threaded EVM benchmarks

EVM execution is single-threaded. With -N (auto-detect), GHC creates one
capability per core, each with a 32MB nursery. The extra capabilities add
GC synchronization overhead without benefit since only one is used.

Also adds -rtsopts so RTS parameters can be overridden at runtime.

Author: elopez

hevm.cabal

@@ -381,7 +381,7 @@ benchmark bench-perf
   hs-source-dirs:
     bench
   ghc-options:
-    -O2 -threaded "-with-rtsopts=-N -A32m" -fproc-alignment=64
+    -O2 -threaded -rtsopts "-with-rtsopts=-N1 -A32m" -fproc-alignment=64
   other-modules:
     Paths_hevm
   autogen-modules: