Change #if DEF to #if defined(DEF)

With the fix for JDK-8211146 several C-preprocessor statements of form #elif __linux__ were changed to more accurate #elif defined(__linux__).

grep found a few more occurrences of the same pattern.

For example (not a complete list):

diff --git a/src/java.base/unix/native/libnio/ch/SocketChannelImpl.c b/src/java.base/unix/native/libnio/ch/SocketChannelImpl.c
--- a/src/java.base/unix/native/libnio/ch/SocketChannelImpl.c
+++ b/src/java.base/unix/native/libnio/ch/SocketChannelImpl.c
@@ -31,7 +31,7 @@
 #include <string.h>
 #include <poll.h>
.
-#if __linux__
+#if defined(__linux__)
 #include <netinet/in.h>
 #endif
.
diff --git a/src/jdk.sctp/unix/native/libsctp/SctpNet.c b/src/jdk.sctp/unix/native/libsctp/SctpNet.c
--- a/src/jdk.sctp/unix/native/libsctp/SctpNet.c
+++ b/src/jdk.sctp/unix/native/libsctp/SctpNet.c
@@ -400,7 +400,7 @@
     int i, addrCount;
     jobjectArray isaa;
.
-#if __solaris__
+#if defined(__solaris__)
     if ((addrCount = nio_sctp_getpaddrs(fd, id, (void **)&addr_buf)) == -1) {
 #else /* __linux__ */
     if ((addrCount = nio_sctp_getpaddrs(fd, id, (struct sockaddr **)&addr_buf)) == -1) {