Rename main to rust_main to support testing

On testing, `std` is imported, which defines an own `main` function. Thus `cargo test` fails if there alread exists one non-mangled main.

src/arch/x86_64/long_mode_init.asm

@@ -13,7 +13,7 @@
 ; limitations under the License.
 
 global long_mode_start
-extern main
+extern rust_main
 
 section .text
 bits 64
@@ -21,7 +21,7 @@ long_mode_start:
     call setup_SSE
 
     ; call rust main
-    call main
+    call rust_main
 .os_returned:
     ; rust main returned, print `OS returned!`
     mov rax, 0x4f724f204f534f4f

src/lib.rs

@@ -21,7 +21,7 @@ extern crate rlibc;
 use core::intrinsics::offset;
 
 #[no_mangle]
-pub extern fn main() {
+pub extern fn rust_main() {
     // ATTENTION: we have a very small stack and no guard page
     let x = ["Hello", " ", "World", "!"];
     let screen_pointer = 0xb8000 as *const u16;