[zion] POC for defining syscalls using macros

zion/usr/crt0.s

@@ -8,6 +8,7 @@ _start:
 _exit:
   // EXIT syscall.
   mov $1, %rdi
-  // Return code as a param.
-  mov %rax, %rsi
+  // Return code as a "struct" on the stack.
+  push %rax
+  mov %rsp, %rsi
   syscall

zion/usr/zcall.cpp

@@ -19,10 +19,6 @@ z_err_t SysCall1(uint64_t number, const void* first) {
   return SysCall2(number, first, 0);
 }
 
-void ZProcessExit(uint64_t code) {
-  SysCall1(Z_PROCESS_EXIT, reinterpret_cast<void*>(code));
-}
-
 z_err_t ZProcessSpawn(z_cap_t proc_cap, z_cap_t bootstrap_cap,
                       z_cap_t* new_proc_cap, z_cap_t* new_vmas_cap,
                       z_cap_t* new_bootstrap_cap) {
@@ -38,16 +34,6 @@ z_err_t ZProcessSpawn(z_cap_t proc_cap, z_cap_t bootstrap_cap,
   return ret;
 }
 
-z_err_t ZThreadCreate(z_cap_t proc_cap, z_cap_t* thread_cap) {
-  ZThreadCreateReq req{
-      .proc_cap = proc_cap,
-  };
-  ZThreadCreateResp resp;
-  z_err_t ret = SysCall2(Z_THREAD_CREATE, &req, &resp);
-  *thread_cap = resp.thread_cap;
-  return ret;
-}
-
 z_err_t ZThreadStart(z_cap_t thread_cap, uint64_t entry, uint64_t arg1,
                      uint64_t arg2) {
   ZThreadStartReq req{

zion/usr/zcall_internal.h

@@ -15,14 +15,6 @@ struct ZProcessSpawnResp {
   z_cap_t bootstrap_cap;
 };
 
-struct ZThreadCreateReq {
-  z_cap_t proc_cap;
-};
-
-struct ZThreadCreateResp {
-  z_cap_t thread_cap;
-};
-
 struct ZThreadStartReq {
   z_cap_t thread_cap;
   uint64_t entry;