[TESTBUG] -Wdeprecated-declarations is reported at sigset() in exesigtest.c

`sigset()` is deprecated, and `__attribute_deprecated_msg__` has been set to the declaration in glibc [1]

We can see the warning on `make test-image` as below:

* Fedora 33 x86_64
    * gcc: gcc-10.2.1-6.fc33.x86_64
    * glibc: glibc-2.32-2.fc33.x86_64

```
/home/ysuenaga/github-forked/jdk/test/hotspot/jtreg/runtime/signal/exesigtest.c: In function 'setSignalHandler':
/home/ysuenaga/github-forked/jdk/test/hotspot/jtreg/runtime/signal/exesigtest.c:245:9: error: 'sigset' is deprecated: Use the signal and sigprocmask functions instead [-Werror=deprecated-declarations]
  245 | sigset(signal_num, handler);
      | ^~~~~~
In file included from /home/ysuenaga/github-forked/jdk/test/hotspot/jtreg/runtime/signal/exesigtest.c:25:
/usr/include/signal.h:353:23: note: declared here
  353 | extern __sighandler_t sigset (int __sig, __sighandler_t __disp) __THROW
      | ^~~~~~
cc1: all warnings being treated as errors
gmake[3]: *** [test/JtregNativeHotspot.gmk:1525: /home/ysuenaga/github-forked/jdk/build/linux-x86_64-server-fastdebug/support/test/hotspot/jtreg/native/support/exesigtest/exesigtest.o] Error 1
gmake[3]: *** Waiting for unfinished jobs....
gmake[2]: *** [make/Main.gmk:612: build-test-hotspot-jtreg-native] Error 2
```

[1] https://sourceware.org/git/?p=glibc.git;a=commit;f=signal/signal.h;h=02802fafcf6e11ea3f998f685035ffe568dfddeb