Use aapcs for efiapi calling convention on arm

Use aapcs for efiapi calling convention on arm

On arm, llvm treats the C calling convention as `aapcs` on soft-float
targets and `aapcs-vfp` on hard-float targets [1]. UEFI specifies in the
arm calling convention that floating point extensions aren't used [2],
so always translate `efiapi` to `aapcs` on arm.

[1]: https://github.com/rust-lang/compiler-builtins/issues/116#issuecomment-261057422
[2]: https://uefi.org/specs/UEFI/2.10/02_Overview.html#detailed-calling-convention

https://github.com/rust-lang/rust/issues/65815