Revert "[NFCI][lldb][test] Avoid unnecessary GNU extension for assembly call" (llvm#166970)

Reverts llvm#166769

Darwin platforms prefix symbols with `_`, other platforms don't
necessarily.

Author: bulbazord

lldb/test/Shell/Unwind/Inputs/call-asm.c

@@ -1,2 +1,3 @@
-int asm_main();
+int asm_main() asm("asm_main");
+
 int main() { return asm_main(); }