fix: remove "C" calling convention specification for vmexit_trampoline and run_guest functions

Author: ZR233

src/exception.rs

@@ -340,7 +340,7 @@ fn current_el_sync_handler(tf: &mut TrapFrame) {
 ///   invoked as part of the low-level hypervisor or VM exit handling routines.
 #[unsafe(naked)]
 #[unsafe(no_mangle)]
-unsafe extern "C" fn vmexit_trampoline() -> ! {
+unsafe extern fn vmexit_trampoline() -> ! {
     core::arch::naked_asm!(
         // Curretly `sp` points to the base address of `Aarch64VCpu.ctx`, which stores guest's `TrapFrame`.
         "add x9, sp, 34 * 8", // Skip the exception frame.

src/pcpu.rs

@@ -23,7 +23,7 @@ static ORI_EXCEPTION_VECTOR_BASE: usize = 0;
 #[percpu::def_percpu]
 pub static IRQ_HANDLER: OnceCell<&(dyn Fn() + Send + Sync)> = OnceCell::new();
 
-unsafe extern "C" {
+unsafe extern {
     fn exception_vector_base_vcpu();
 }
 

src/vcpu.rs

@@ -215,7 +215,7 @@ impl<H: AxVCpuHal> Aarch64VCpu<H> {
     /// When a VM-Exit happens when guest's vCpu is running,
     /// the control flow will be redirected to this function through `return_run_guest`.
     #[unsafe(naked)]
-    unsafe extern "C" fn run_guest(&mut self) -> usize {
+    unsafe extern fn run_guest(&mut self) -> usize {
         // Fixes: https://github.com/arceos-hypervisor/arm_vcpu/issues/22
         //
         // The original issue seems to be caused by an unexpected compiler optimization that takes