[singlejar] Add port library for various MSVC hacks

A small `port` library with various MSVC hacks. This is needed for future PRs.

For non-Windows platforms, `port.cc` will produce an empty object file.

#2241 /cc @laszlocsomor

Closes #5501.

PiperOrigin-RevId: 204889516
diff --git a/src/tools/singlejar/BUILD b/src/tools/singlejar/BUILD
index a855a220..5c1da1f 100644
--- a/src/tools/singlejar/BUILD
+++ b/src/tools/singlejar/BUILD
@@ -31,6 +31,8 @@
         "options.h",
         "output_jar.cc",
         "output_jar.h",
+        "port.cc",
+        "port.h",
         "singlejar_main.cc",
         "token_stream.h",
         "transient_bytes.h",
@@ -345,6 +347,13 @@
 )
 
 cc_library(
+    name = "port",
+    srcs = ["port.cc"],
+    hdrs = ["port.h"],
+    visibility = ["//visibility:private"],
+)
+
+cc_library(
     name = "mapped_file",
     srcs = select({
         "//src:windows": ["mapped_file_windows.inc"],