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

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.