[ELF] Add .got.plt to PT_GNU_RELRO if -z now

If external symbols are resolved eagerly on process startup, .got.plt
can be read-only and therefore can be in the PT_GNU_RELRO segment.

elf/output-chunks.cc

@@ -124,6 +124,7 @@ bool is_relro(Context<E> &ctx, Chunk<E> *chunk) {
     if ((flags & SHF_TLS) || type == SHT_INIT_ARRAY ||
         type == SHT_FINI_ARRAY || type == SHT_PREINIT_ARRAY ||
         chunk == ctx.got.get() || chunk == ctx.dynamic.get() ||
+        (ctx.arg.z_now && chunk == ctx.gotplt.get()) ||
         chunk->name.ends_with(".rel.ro"))
       return true;
   return false;

test/elf/relro.sh

@@ -11,15 +11,25 @@ t=out/test/elf/$testname
 mkdir -p $t
 
 cat <<EOF | $CC -c -xc -o $t/a.o -
-int main() {}
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
 EOF
 
-$CC -B. -o $t/exe $t/a.o
-readelf --segments -W $t/exe > $t/log
-grep -q 'GNU_RELRO ' $t/log
+$CC -B. -o $t/exe1 $t/a.o -Wl,-z,relro,-z,lazy
+$t/exe1 | grep -q 'Hello world'
+readelf --segments -W $t/exe1 > $t/log1
+grep -q 'GNU_RELRO ' $t/log1
 
-$CC -B. -o $t/exe $t/a.o -Wl,-z,norelro
-readelf --segments -W $t/exe > $t/log
-! grep -q 'GNU_RELRO ' $t/log || false
+$CC -B. -o $t/exe2 $t/a.o -Wl,-z,relro,-z,now
+$t/exe2 | grep -q 'Hello world'
+readelf --segments -W $t/exe2 > $t/log2
+grep -q 'GNU_RELRO ' $t/log2
+
+$CC -B. -o $t/exe3 $t/a.o -Wl,-z,norelro
+$t/exe3 | grep -q 'Hello world'
+readelf --segments -W $t/exe3 > $t/log3
+! grep -q 'GNU_RELRO ' $t/log3 || false
 
 echo OK