x86: Change __kernel_size_t conditionals to use compiler provided defines

Since commit bb0bb91c ("efi_stub: Use efi_uintn_t"), EFI x86
64-bit payload does not work anymore. The call to GetMemoryMap()
in efi_stub.c fails with return code EFI_INVALID_PARAMETER. Since
the payload itself is still 32-bit U-Boot, efi_uintn_t gets wrongly
interpreted as int, but it should actually be long in a 64-bit EFI
environment.

This changes the x86 __kernel_size_t conditionals to use compiler
provided defines instead. That way we always adhere to the build
environment we're in and the definitions adjust automatically.

Fixes: bb0bb91c ("efi_stub: Use efi_uintn_t")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>