efi_loader: access __efi_runtime_start/stop without &

A symbol defined in a linker script (e.g. __efi_runtime_start = .;) is
only a symbol, not a variable and should not be dereferenced.
The common practice is either define it as
extern uint32_t __efi_runtime_start or
extern char __efi_runtime_start[] and access it as
&__efi_runtime_start or __efi_runtime_start respectively.

So let's access it properly since we define it as an array

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>