efi_loader: Always use EFIAPI instead of asmlinkage

EFI calls are usually defined as asmlinkage. That means we pass all parameters
to functions via the stack x86_32.

On x86_64 however, we need to also stick to the MS ABI calling conventions,
which the EFIAPI define conveniently handles for us. Most EFI functions were
also marked with EFIAPI, except for the entry call.

So this patch adjusts all entry calls to use EFIAPI instead of the manual
asmlinkage attribute.

While at it, we also change the prototype of the entry point to return
efi_status_t instead of ulong, as this is the correct prototype definition.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - Use efi_status_t in all occurences