FEXInterpreter: Punch through a /sys/fex/rootfs node

Currently pressure-vessel does a bit of a bodge to get the x86 rootfs
path when running inside of FEX. It does this by opening `/.` which
works around our pseudo-overlayfs tracking. While this worked, it wasn't
guaranteed to work forever. With FEX-Emu#4225 working to fix more issues around
how rootfs is laid out, it had to break this path (while adding a
workaround for it to keep working).

Give pressure-vessel a blessed path from the EmulatedFiles code paths to
get a real fd back to the x86 rootfs, that way if we break this code
path it is entirely our problem to fix.

Still need to have a conversation with upstream pressure-vessel to see
if they'll accept this path or if we need to do something different.

Source/Tools/LinuxEmulation/LinuxSyscalls/EmulatedFiles/EmulatedFiles.cpp

@@ -573,6 +573,12 @@ EmulatedFDManager::EmulatedFDManager(FEXCore::Context::Context* ctx)
   fextl::string procCmdLine = fextl::fmt::format("/proc/{}/cmdline", getpid());
   FDReadCreators[procCmdLine] = cmdline_handler;
 
+  // FEX-Emu specific file handles
+  FDReadCreators["/sys/fex/rootfs"] = [&](FEXCore::Context::Context* ctx, int32_t fd, const char* pathname, int32_t flags, mode_t mode) -> int32_t {
+    // Redirect to the guest rootfs
+    return ::syscall(SYSCALL_DEF(openat), fd, LDPath().c_str(), flags, mode);
+  };
+
   if (ThreadsConfig > 1) {
     cpus_online = fextl::fmt::format("0-{}", ThreadsConfig - 1);
   } else {

Source/Tools/LinuxEmulation/LinuxSyscalls/EmulatedFiles/EmulatedFiles.h

@@ -35,5 +35,7 @@ class EmulatedFDManager {
 
   static int32_t ProcAuxv(FEXCore::Context::Context* ctx, int32_t fd, const char* pathname, int32_t flags, mode_t mode);
   const uint32_t ThreadsConfig;
+
+  FEX_CONFIG_OPT(LDPath, ROOTFS);
 };
 } // namespace FEX::EmulatedFile