MJIT: Self-contain extern declarations

I'm trying to get rid of mjit_compiler.c, and this helps that migration.

mjit_c.rb

@@ -51,7 +51,10 @@ module RubyVM::MJIT
 
     def rb_splat_or_kwargs_p(ci)
       _ci_addr = ci.to_i
-      Primitive.cexpr! 'RBOOL(rb_splat_or_kwargs_p((CALL_INFO)NUM2PTR(_ci_addr)))'
+      Primitive.cstmt! %{
+        extern bool rb_splat_or_kwargs_p(const struct rb_callinfo *restrict ci);
+        return RBOOL(rb_splat_or_kwargs_p((CALL_INFO)NUM2PTR(_ci_addr)));
+      }
     end
 
     def fastpath_applied_iseq_p(ci, cc, iseq)
@@ -74,7 +77,10 @@ module RubyVM::MJIT
     def mjit_capture_cc_entries(compiled_body, captured_body)
       _compiled_body_addr = compiled_body.to_i
       _captured_body_addr = captured_body.to_i
-      Primitive.cexpr! 'INT2NUM(mjit_capture_cc_entries((struct rb_iseq_constant_body *)NUM2PTR(_compiled_body_addr), (struct rb_iseq_constant_body *)NUM2PTR(_captured_body_addr)))'
+      Primitive.cstmt! %{
+        extern int mjit_capture_cc_entries(const struct rb_iseq_constant_body *compiled_iseq, const struct rb_iseq_constant_body *captured_iseq);
+        return INT2NUM(mjit_capture_cc_entries((struct rb_iseq_constant_body *)NUM2PTR(_compiled_body_addr), (struct rb_iseq_constant_body *)NUM2PTR(_captured_body_addr)));
+      }
     end
 
     # Convert encoded VM pointers to insn BINs.

mjit_compiler.c

@@ -56,9 +56,6 @@ fastpath_applied_iseq_p(const CALL_INFO ci, const CALL_CACHE cc, const rb_iseq_t
 #define PTR2NUM(x) ULL2NUM(x)
 #endif
 
-extern int
-mjit_capture_cc_entries(const struct rb_iseq_constant_body *compiled_iseq, const struct rb_iseq_constant_body *captured_iseq);
-
 // Compile ISeq to C code in `f`. It returns true if it succeeds to compile.
 bool
 mjit_compile(FILE *f, const rb_iseq_t *iseq, const char *funcname, int id)
@@ -103,8 +100,6 @@ has_cache_for_send(rb_execution_context_t *ec, VALUE self, VALUE cc_addr, VALUE
     return RBOOL(has_cache);
 }
 
-extern bool rb_splat_or_kwargs_p(const struct rb_callinfo *restrict ci);
-
 // An offsetof implementation that works for unnamed struct and union.
 // Multiplying 8 for compatibility with libclang's offsetof.
 #define OFFSETOF(ptr, member) RB_SIZE2NUM(((char *)&ptr.member - (char*)&ptr) * 8)