FreeBSD: Enable a few more C++ interop tests

Some of these tests were marked as unsupported since they were only
checking for macOS and Linux. They seem to be passing on FreeBSD as
well, so enabling them here.

(cherry picked from commit 3a48d6e)

Author: etcwilde

test/Interop/Cxx/stdlib/avoid-import-cxx-math.swift

@@ -2,7 +2,7 @@
 // RUN: %target-swift-frontend %s -typecheck -verify -cxx-interoperability-mode=swift-6
 // RUN: %target-swift-frontend %s -typecheck -verify -cxx-interoperability-mode=upcoming-swift
 
-// REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: OS=macosx || OS=linux-gnu || OS=freebsd
 
 import CxxStdlib
 

test/Interop/Cxx/stdlib/overlay/custom-sequence.swift

@@ -1,7 +1,7 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
 //
 // REQUIRES: executable_test
-// REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: OS=macosx || OS=linux-gnu || OS=freebsd
 
 // REQUIRES: rdar102364960
 

test/Interop/Cxx/stdlib/use-std-map.swift

@@ -11,7 +11,7 @@
 
 // REQUIRES: executable_test
 //
-// REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: OS=macosx || OS=linux-gnu || OS=freebsd
 
 import StdlibUnittest
 #if !BRIDGING_HEADER