[solaris/illumos] add SIGRTMIN and SIGRTMAX

Add these functions, similar to the Linux ones. Also add tests.

For illumos, the source code is at [1] and documentation is at [2]. Blame
suggests that Solaris also supports the same calls.

[1]: https://github.com/illumos/illumos-gate/blame/27ecbff00d8c86a2647d6fe325cacb220d712115/usr/src/uts/common/sys/iso/signal_iso.h#L100-L101
[2]: https://illumos.org/man/3HEAD/signal.h

(backport <rust-lang#4171>)
(cherry picked from commit a8aaadf)

libc-test/build.rs

@@ -42,6 +42,8 @@ fn do_cc() {
         || target.contains("l4re")
         || target.contains("android")
         || target.contains("emscripten")
+        || target.contains("solaris")
+        || target.contains("illumos")
     {
         cc::Build::new().file("src/sigrt.c").compile("sigrt");
     }

libc-test/semver/solarish.txt

@@ -16,6 +16,8 @@ LIO_WAIT
 LIO_WRITE
 PIPE_BUF
 SIGEV_PORT
+SIGRTMAX
+SIGRTMIN
 _POSIX_VDISABLE
 _ST_FSTYPSZ
 aio_cancel

libc-test/test/sigrt.rs

@@ -4,7 +4,9 @@
     target_os = "linux",
     target_os = "l4re",
     target_os = "android",
-    target_os = "emscripten"
+    target_os = "emscripten",
+    target_os = "solaris",
+    target_os = "illumos",
 ))]
 mod t {
     use libc;

src/unix/solarish/mod.rs

@@ -2542,6 +2542,14 @@ f! {
 }
 
 safe_f! {
+    pub fn SIGRTMAX() -> c_int {
+        unsafe { crate::sysconf(_SC_SIGRT_MAX) as c_int }
+    }
+
+    pub fn SIGRTMIN() -> c_int {
+        unsafe { crate::sysconf(_SC_SIGRT_MIN) as c_int }
+    }
+
     pub {const} fn WIFEXITED(status: c_int) -> bool {
         (status & 0xFF) == 0
     }