[gfortran] Add ACCESS="APPEND" to OPEN statement

[François-Xavier Coudert]

Attached patch fixes PR libfortran/24892: g77 allows the ACCESS
keywork of the OPEN statement to have value "APPEND", which is the
same as POSITION="APPEND". The patch adds this possibility for
compatibility reasons, warning if the code was compiled with
-pedantic, and refusing the cases of conflicting ACCESS and POSITION
keywords.

Tested on ppc-darwin, regtesting still going on.

OK for mainline? OK for 4.0?

:ADDPATCH fortran:
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