[openacc] Ensure oacc_replace_fn_attrib replaces

Atm, in oacc_replace_fn_attrib we only replace the oacc function attribute if
it's the first.

So, for a parallel region we have:
...
__attribute__((oacc function (, 20, ), omp target entrypoint))
...
which is replaced by:
...
__attribute__((oacc function (1, 20, 32), omp target entrypoint))
...

But for a routine:
...
__attribute__((omp declare target, oacc function (0 1, 0 1, 0 1)))
...
we get instead:
...
__attribute__((oacc function (0 1, 0 1, 0 1), omp declare target,
	       oacc function (0 1, 0 1, 0 1)))
...

Fix this confusing behaviour by ensuring the oacc function attribute is
indeed replaced.

---
 gcc/omp-general.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/gcc/omp-general.c b/gcc/omp-general.c
index 99d8226ef21..e37a6e0b96c 100644
--- a/gcc/omp-general.c
+++ b/gcc/omp-general.c
@@ -545,9 +545,13 @@ oacc_replace_fn_attrib (tree fn, tree dims)
   tree ident = get_identifier (OACC_FN_ATTRIB);
   tree attribs = DECL_ATTRIBUTES (fn);
 
-  /* If we happen to be present as the first attrib, drop it.  */
-  if (attribs && TREE_PURPOSE (attribs) == ident)
-    attribs = TREE_CHAIN (attribs);
+  tree *elem = &attribs;
+  while (*elem && TREE_PURPOSE (*elem) != ident)
+    elem = &TREE_CHAIN (attribs);
+
+  if (*elem)
+    *elem = TREE_CHAIN (*elem);
+
   DECL_ATTRIBUTES (fn) = tree_cons (ident, dims, attribs);
 }