5l/6l/8l: add a DT_DEBUG dynamic tag to a dynamic ELF binary

This requires making the .dynamic section writable, as the
dynamic linker will change the value of the DT_DEBUG tag at
runtime.  The DT_DEBUG tag is used by gdb to find all loaded
shared libraries.

R=rsc
CC=golang-dev
https://golang.org/cl/5189044

src/cmd/5l/asm.c

@@ -232,7 +232,7 @@ doelf(void)
 		/* define dynamic elf table */
 		s = lookup(".dynamic", 0);
 		s->reachable = 1;
-		s->type = SELFROSECT;
+		s->type = SELFSECT; // writable
 
 		/*
 		 * .dynamic table
@@ -251,6 +251,7 @@ doelf(void)
 		elfwritedynent(s, DT_PLTREL, DT_REL);
 		elfwritedynentsymsize(s, DT_PLTRELSZ, lookup(".rel.plt", 0));
 		elfwritedynentsym(s, DT_JMPREL, lookup(".rel.plt", 0));
+		elfwritedynent(s, DT_DEBUG, 0);
 		elfwritedynent(s, DT_NULL, 0);
 	}
 }

src/cmd/6l/asm.c

@@ -649,7 +649,7 @@ doelf(void)
 		/* define dynamic elf table */
 		s = lookup(".dynamic", 0);
 		s->reachable = 1;
-		s->type = SELFROSECT;
+		s->type = SELFSECT; // writable
 
 		/*
 		 * .dynamic table
@@ -670,6 +670,8 @@ doelf(void)
 		elfwritedynentsymsize(s, DT_PLTRELSZ, lookup(".rela.plt", 0));
 		elfwritedynentsym(s, DT_JMPREL, lookup(".rela.plt", 0));
 		
+		elfwritedynent(s, DT_DEBUG, 0);
+
 		// Do not write DT_NULL.  elfdynhash will finish it.
 	}
 }

src/cmd/8l/asm.c

@@ -607,7 +607,7 @@ doelf(void)
 		/* define dynamic elf table */
 		s = lookup(".dynamic", 0);
 		s->reachable = 1;
-		s->type = SELFROSECT;
+		s->type = SELFSECT; // writable
 
 		/*
 		 * .dynamic table
@@ -627,6 +627,8 @@ doelf(void)
 		elfwritedynentsymsize(s, DT_PLTRELSZ, lookup(".rel.plt", 0));
 		elfwritedynentsym(s, DT_JMPREL, lookup(".rel.plt", 0));
 
+		elfwritedynent(s, DT_DEBUG, 0);
+
 		// Do not write DT_NULL.  elfdynhash will finish it.
 	}
 }