arm: Replace 'adrl' in EFI crt0

arm: Replace 'adrl' in EFI crt0

LLVM's IAS does not (and cannot easily) support the 'adrl'
pseudoinstruction, and ARM developers generally do not consider it
portable across assembler implementations either.

Instead, expand it into the two subtract instructions it would emit
anyway. An explanation of the math follows:

The .+8 and .+4 refer to the same memory location; this is because the
.+4 expression occurs in a subsequent instruction, 4 bytes after the
first. This memory location is the value of the PC register when it is
read by the first sub instruction. Thus, both inner parenthesized
expressions evaluate to the same result: PC's offset relative to
image_base. The subtract instructions then remove one byte each
(low, then high) of the total offset, thereby getting the absolute
address of image_base loaded in r0.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>