Allow rules to override -O2 with their own compiler_flags.

Currently, `-c opt` will turn on optimization for all Haskell
rules with no way to disable it.  Previously, it was possible
to do so with `compiler_flags = ["-O0"]`, but that behavior was
changed in #342:
https://github.com/tweag/rules_haskell/pull/342/files#diff-33a8c46955e2a9917090b5258346aee2L247

Some Haskell packages take a *lot* of memory and a lot longer to compile with
optimizations.  So it's useful for us to be able to control this tradeoff.

haskell/private/actions/compile.bzl

@@ -236,6 +236,11 @@ def _compilation_defaults(hs, cc, java, dep_info, srcs, extra_srcs, cpp_defines,
 
   # Default compiler flags.
   ghc_args += hs.toolchain.compiler_flags
+
+  # Compilation mode.  Allow rule-supplied compiler flags to override it.
+  if hs.mode == "opt":
+    ghc_args += ["-O2"]
+
   ghc_args += compiler_flags
   ghc_args.append("-hide-all-packages")
 
@@ -426,10 +431,6 @@ def _compilation_defaults(hs, cc, java, dep_info, srcs, extra_srcs, cpp_defines,
   args = hs.actions.args()
   args.add(ghc_args)
 
-  # Compilation mode and explicit user flags
-  if hs.mode == "opt":
-    args.add("-O2")
-
   args.add(["-static"])
 
   # NOTE We can't have profiling and dynamic code at the same time, see: