Windows LTO build fails
Description of problem:
LTO likes to delete `win32_close_exception_handler` which causes an error when linking
```
[2736/5786] Linking target qemu-system-avr.exe
FAILED: qemu-system-avr.exe
"cc" "-m64" "-mcx16" @qemu-system-avr.exe.rsp
`win32_close_exception_handler' referenced in section `.xdata' of C:\msys64\tmp\cceRwR4N.ltrans59.ltrans.o: defined in discarded section `.text' of libqemuutil.a.p/util_oslib-win32.c.obj (symbol from plugin)
collect2.exe: error: ld returned 1 exit status
```
Steps to reproduce:
1. `./configure --enable-lto`
2. `make`
Additional information:
Looks like the offending commit is d89f30b4df13dfe389a4d6cf8a30b2f87c4c166e "win32: wrap socket close() with an exception handler".
Undoing the commit or marking the exception handler as `__attribute__ ((noinline, used))` both appear to fix the issue.