cmd/ld: better diagnostics on unaligned symbols

Want to see why builders are failing.
Then decide whether to rollback or fix.

TBR=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/114510043

src/cmd/ld/data.c

@@ -815,7 +815,8 @@ proggenaddsym(ProgGen *g, LSym *s)
 	if(s->gotype == nil && s->size >= PtrSize) {
 		// conservative scan
 		if((s->size%PtrSize) || (g->pos%PtrSize))
-			diag("proggenaddsym: unaligned symbol");
+			diag("proggenaddsym: unaligned conservative symbol %s: size=%lld pos=%lld",
+				s->name, s->size, g->pos);
 		size = (s->size+PtrSize-1)/PtrSize*PtrSize;
 		if(size < 32*PtrSize) {
 			// Emit small symbols as data.
@@ -840,7 +841,8 @@ proggenaddsym(ProgGen *g, LSym *s)
 		} else {
 			// Emit large symbols as array.
 			if((s->size%PtrSize) || (g->pos%PtrSize))
-				diag("proggenaddsym: unaligned symbol");
+				diag("proggenaddsym: unaligned noscan symbol %s: size=%lld pos=%lld",
+					s->name, s->size, g->pos);
 			proggenarray(g, s->size/PtrSize);
 			proggendata(g, BitsScalar);
 			proggenarrayend(g);
@@ -852,7 +854,8 @@ proggenaddsym(ProgGen *g, LSym *s)
 		gcprog = decodetype_gcprog(s->gotype);
 		size = decodetype_size(s->gotype);
 		if((size%PtrSize) || (g->pos%PtrSize))
-			diag("proggenaddsym: unaligned symbol");
+			diag("proggenaddsym: unaligned gcprog symbol %s: size=%lld pos=%lld",
+				s->name, s->size, g->pos);
 		for(i = 0; i < gcprog->np-1; i++)
 			proggenemit(g, gcprog->p[i]);
 		g->pos = s->value + size;
@@ -861,7 +864,8 @@ proggenaddsym(ProgGen *g, LSym *s)
 		mask = decodetype_gcmask(s->gotype);
 		size = decodetype_size(s->gotype);
 		if((size%PtrSize) || (g->pos%PtrSize))
-			diag("proggenaddsym: unaligned symbol");
+			diag("proggenaddsym: unaligned gcmask symbol %s: size=%lld pos=%lld",
+				s->name, s->size, g->pos);
 		for(i = 0; i < size; i += PtrSize)
 			proggendata(g, (mask[i/PtrSize/2]>>((i/PtrSize%2)*4+2))&BitsMask);
 		g->pos = s->value + size;