Use efiapi for entry point

See merge request redox-os/uefi_std!5

Enable the ABI feature and use it for the entry point. This will set
calling conventions to what is specified by the UEFI spec.

It currently only sets x86-64 to `win64` and everything else to `C`.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

Fix building on nightly

See merge request !3

Update dependencies and remove raw feature.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

Use efi_main() instead of start() as name for entry point

See merge request !2

This is what the UEFI targets included in Rustc use for the entry point:
https://github.com/rust-lang/rust/blob/master/compiler/rustc_target/src/spec/uefi_msvc_base.rs

With this change, `uefi_std` can be used with the standard target
without passing a `-Z pre-link-arg=` argument to change the entry point
to `_start`.

Remove eh_unwind_resume lang item

See merge request !1

The eh_unwind_resume lang item has been removed [1], which prevents
using uefi_std with newer toolchains.

[1]: https://github.com/rust-lang/rust/pull/67502