[bazel] Replace features with explicit flags in coremark lib

Matches the arguments used in `//third_party/coremark/top_earlgrey`.
Allows the feature-per-warning method from CRT to be replaced with one
set of warnings which is easier to manage.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

Author: jwnrt

third_party/coremark/BUILD.coremark.bazel

@@ -16,9 +16,12 @@ cc_library(
     hdrs = [
         "coremark.h",
     ],
-    features = [
-        "-implicit_conversion_warnings",
-        "-strict_prototypes_warnings",
+    copts = [
+        "-Wno-implicit-fallthrough",
+        "-Wno-strict-prototypes",
+        "-Wno-implicit-int-conversion",
+        "-Wno-sign-conversion",
+        "-Wno-shorten-64-to-32",
     ],
     includes = ["."],
     deps = [