[FREELDR] Change real mode base address (reactos#7530)

Now that's our FRLDR16 base address is zero
all the **INTERNAL** variables and functions have to
use offsets instead of absolute addresses.

Here's the example:
```
// CS = HEX(7C0)
// MyLabel = HEX(100)

lea eax, dword ptr cs:[MyLabel] // EAX = ((CS << 4) + offset MyLabel) = 7D00
```

Author: iLauncherDev

boot/freeldr/freeldr/pcat.cmake

@@ -13,12 +13,12 @@ if(ARCH STREQUAL "i386")
     CreateBootSectorTarget(frldr16
         ${CMAKE_CURRENT_SOURCE_DIR}/arch/realmode/i386.S
         ${CMAKE_CURRENT_BINARY_DIR}/frldr16.bin
-        F800)
+        0000)
 elseif(ARCH STREQUAL "amd64")
     CreateBootSectorTarget(frldr16
         ${CMAKE_CURRENT_SOURCE_DIR}/arch/realmode/amd64.S
         ${CMAKE_CURRENT_BINARY_DIR}/frldr16.bin
-        F800)
+        0000)
 endif()