illumos: expose PIPE_BUF constant

(backport <#3633>)
(cherry picked from commit 64d8a2f)

Author: dancrossnyc

libc-test/semver/solarish.txt

@@ -0,0 +1 @@
+PIPE_BUF

src/unix/solarish/mod.rs

@@ -1279,6 +1279,7 @@ pub const FOPEN_MAX: ::c_uint = 20;
 pub const FILENAME_MAX: ::c_uint = 1024;
 pub const L_tmpnam: ::c_uint = 25;
 pub const TMP_MAX: ::c_uint = 17576;
+pub const PIPE_BUF: ::c_int = 5120;
 
 pub const GRND_NONBLOCK: ::c_uint = 0x0001;
 pub const GRND_RANDOM: ::c_uint = 0x0002;