x86: Change __kernel_size_t conditionals to use compiler provided defines

Since commit bb0bb91cf0 ("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: bb0bb91cf0 ("efi_stub: Use efi_uintn_t")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/x86/include/asm/posix_types.h

@@ -16,7 +16,8 @@ typedef int		__kernel_pid_t;
 typedef unsigned short	__kernel_ipc_pid_t;
 typedef unsigned short	__kernel_uid_t;
 typedef unsigned short	__kernel_gid_t;
-#if CONFIG_IS_ENABLED(X86_64)
+/* checking against __x86_64__ covers both 64-bit EFI stub and 64-bit U-Boot */
+#if defined(__x86_64__)
 typedef unsigned long	__kernel_size_t;
 typedef long		__kernel_ssize_t;
 #else