rt: Omit leading underscore on symbol names on Linux

src/rt/arch/i386/ccall.S

@@ -4,8 +4,13 @@
 //
 // Note that we could use |enter| and |leave| but the manuals tell me they're
 // slower.
+#if defined(__APPLE__) || defined(_WIN32)
 .globl _upcall_call_c_stack
 _upcall_call_c_stack:
+#else
+.globl upcall_call_c_stack
+upcall_call_c_stack:
+#endif
     pushl %ebp
     movl %esp,%ebp          // save esp
     movl 8(%esp),%eax       // eax = callee