libgolang: Fix build with old gcc

Gcc 5.4 from Ubuntu 16.04 LTS complains e.g.

    ./golang/libgolang.h: In function ‘golang::_selcase golang::_selrecv_(golang::_chan*, void*, bool*)’:
    ./golang/libgolang.h:227:5: sorry, unimplemented: non-trivial designated initializers not supported
         };
         ^

The problem started to appear after 47111d3e (libgolang: Teach select to accept
inplace tx data) when we moved _selcase.ptxrx inside union.

Let's add workaround for older compilers, even though e.g. gcc 8.3 from Debian
10 accepts existing code just fine.

golang/libgolang.h

@@ -194,9 +194,9 @@ _selcase _selsend(_chan *ch, const void *ptx) {
         .op     = _CHANSEND,
         .flags  = (enum _selflags)0,
         .user   = 0xff,
-        .ptxrx  = (void *)ptx,
-        .rxok   = NULL,
     };
+    _   .ptxrx  = (void *)ptx;
+    _   .rxok   = NULL;
     return _;
 }
 
@@ -208,9 +208,9 @@ _selcase _selrecv(_chan *ch, void *prx) {
         .op     = _CHANRECV,
         .flags  = (enum _selflags)0,
         .user   = 0xff,
-        .ptxrx  = prx,
-        .rxok   = NULL,
     };
+    _   .ptxrx  = prx;
+    _   .rxok   = NULL;
     return _;
 }
 
@@ -222,9 +222,9 @@ _selcase _selrecv_(_chan *ch, void *prx, bool *pok) {
         .op     = _CHANRECV,
         .flags  = (enum _selflags)0,
         .user   = 0xff,
-        .ptxrx  = prx,
-        .rxok   = pok,
     };
+    _   .ptxrx  = prx;
+    _   .rxok   = pok;
     return _;
 }
 

golang/runtime/libgolang.cpp

@@ -842,14 +842,18 @@ void _chan::_dataq_popleft(void *prx) {
 // ---- select ----
 
 // _default represents default case for _select.
-const _selcase _default = {
+static _selcase _mkdefault() {
+    _selcase _ = {
         .ch     = NULL,
         .op     = _DEFAULT,
         .flags  = (_selflags)0,
         .user   = 0xff,
-    .ptxrx  = NULL,
-    .rxok   = NULL,
-};
+    };
+    _   .ptxrx  = NULL;
+    _   .rxok   = NULL;
+    return _;
+}
+const _selcase _default = _mkdefault();
 
 const void *_selcase::ptx() const {
     const _selcase *cas = this;