FEXLoader: Allocate first 4GB of the 64-bit address space

Spurred on by FEX-Emu#3421. To ensure that applications don't take advantage of
small address wrap around, allocate the first 4GB in the 64-bit space.

Some context. Linux always reserves the first 16KB of virtual address
space (unless you tinker with some settings which nobody should do).

Example of 32-bit code:
lea eax, [0xffff_0000]
mov ebx, [eax + 0x1_0000]

The address calculated by the mov will wrap around to 0x0 which will
result in SIGSEGV. If FEX messes up zero extensions then it would try to
access 0x1_0000_0000 instead.

This could result in a 32-bit application potentially accessing some FEX
memory instead of crashing.
Add this safety net which will still SIGSEGV and we will be able to see
the crash.

Source/Tools/FEXLoader/FEXLoader.cpp

@@ -417,11 +417,15 @@ int main(int argc, char **argv, char **const envp) {
 
   fextl::unique_ptr<FEX::HLE::MemAllocator> Allocator;
   fextl::vector<FEXCore::Allocator::MemoryRegion> Base48Bit;
+  fextl::vector<FEXCore::Allocator::MemoryRegion> Low4GB;
 
   if (Loader.Is64BitMode()) {
     // Destroy the 48th bit if it exists
     Base48Bit = FEXCore::Allocator::Steal48BitVA();
   } else {
+    constexpr uint64_t First64BitAddr = 0x1'0000'0000ULL;
+    Low4GB = FEXCore::Allocator::StealMemoryRegion(First64BitAddr, First64BitAddr + First64BitAddr);
+
     // Setup our userspace allocator
     FEXCore::Allocator::SetupHooks();
     Allocator = FEX::HLE::CreatePassthroughAllocator();
@@ -578,6 +582,8 @@ int main(int argc, char **argv, char **const envp) {
 
   FEXCore::Allocator::ClearHooks();
   FEXCore::Allocator::ReclaimMemoryRegion(Base48Bit);
+  FEXCore::Allocator::ReclaimMemoryRegion(Low4GB);
+
   // Allocator is now original system allocator
   FEXCore::Telemetry::Shutdown(Program.ProgramName);
   FEXCore::Profiler::Shutdown();