Make improvements

- Fix unused local variable errors
- Remove yoinks from sigaction() header
- Add nox87 and aarch64 to github actions
- Fix cosmocc -fportcosmo in linking mode
- It's now possible to build `make m=llvm o/llvm/libc`

libc/runtime/winmain.greg.c

@@ -154,6 +154,7 @@ __msabi static textwindows wontreturn void WinMainNew(const char16_t *cmdline) {
                 : (DescribeNtConsoleInFlags)(inflagsbuf, kConsoleModes[i]),
               rc);
     }
+    (void)rc;
   }
   _Static_assert(sizeof(struct WinArgs) % FRAMESIZE == 0, "");
   _mmi.p = _mmi.s;