Split compile and link for MinGW support

MinGW test_jit fails with no error message. Perhaps linker flags should
not be passed when compilation is happening.

Anyway splitting these stages doesn't matter for performance. So let me
just split it to fix the issue. Probably this helps Solaris's issue too.

mjit_worker.c

@@ -855,30 +855,45 @@ make_pch(void)
 }
 
 // Compile .c file to .so file. It returns true if it succeeds. (non-mswin)
+// Not compiling .c to .so directly because it fails on MinGW.
 static bool
 compile_c_to_so(const char *c_file, const char *so_file)
 {
-    const char *options[] = {
-        "-o", so_file, c_file,
+    char* o_file = alloca(strlen(c_file) + 1);
+    strcpy(o_file, c_file);
+    o_file[strlen(c_file) - 1] = 'o';
+
+    const char *o_args[] = {
+        "-o", o_file, c_file,
 # ifdef __clang__
         "-include-pch", pch_file,
 # endif
+        "-c", NULL
+    };
+    char **args = form_args(5, cc_common_args, CC_CODEFLAG_ARGS, cc_added_args, o_args, CC_LINKER_ARGS);
+    if (args == NULL) return false;
+    int exit_code = exec_process(cc_path, args);
+    free(args);
+    if (exit_code != 0) {
+        verbose(2, "compile_c_to_so: failed to compile .c to .o: %d", exit_code);
+        return false;
+    }
+
+    const char *so_args[] = {
+        "-o", so_file,
 # ifdef _WIN32
         libruby_pathflag,
 # endif
-        NULL
+        o_file, NULL
     };
-
-    char **args = form_args(7, CC_LDSHARED_ARGS, CC_CODEFLAG_ARGS, cc_added_args,
-            options, CC_LIBS, CC_DLDFLAGS_ARGS, CC_LINKER_ARGS);
-    if (args == NULL)
-        return false;
-
-    int exit_code = exec_process(cc_path, args);
+    args = form_args(6, CC_LDSHARED_ARGS, CC_CODEFLAG_ARGS, so_args, CC_LIBS, CC_DLDFLAGS_ARGS, CC_LINKER_ARGS);
+    if (args == NULL) return false;
+    exit_code = exec_process(cc_path, args);
     free(args);
-
-    if (exit_code != 0)
-        verbose(2, "compile_c_to_so: compile error: %d", exit_code);
+    if (!mjit_opts.save_temps) remove_file(o_file);
+    if (exit_code != 0) {
+        verbose(2, "compile_c_to_so: failed to link .o to .so: %d", exit_code);
+    }
     return exit_code == 0;
 }
 

test/lib/jit_support.rb

@@ -56,7 +56,7 @@ module JITSupport
     return @supported if defined?(@supported)
     @supported = RbConfig::CONFIG["MJIT_SUPPORT"] != 'no' && UNSUPPORTED_COMPILERS.all? do |regexp|
       !regexp.match?(RbConfig::CONFIG['MJIT_CC'])
-    end && !appveyor_pdb_corrupted? && !PENDING_RUBYCI_NICKNAMES.include?(ENV['RUBYCI_NICKNAME']) && /mingw/ !~ RUBY_PLATFORM # TODO: remove mingw exclusion after investigation
+    end && !appveyor_pdb_corrupted? && !PENDING_RUBYCI_NICKNAMES.include?(ENV['RUBYCI_NICKNAME'])
   end
 
   # AppVeyor's Visual Studio 2013 / 2015 are known to spuriously generate broken pch / pdb, like: