Sync with upstream vhdl mode v3.37.1. Add VHDL'08 support.

Sync with upstream vhdl mode v3.37.1.  Add VHDL'08 support.

* lisp/progmodes/vhdl-mode.el (vhdl-version, vhdl-time-stamp)
(vhdl-doc-release-notes): Update.
(vhdl-standard): Add VHDL'08 option.
(vhdl-sensitivity-list-all): New option.
(vhdl-directive-keywords): Add psl.
(vhdl-offsets-alist-default, vhdl-mode-abbrev-table-init)
(vhdl-template-construct-alist-init, vhdl-create-mode-menu):
(vhdl-imenu-generic-expression): Add context, directive.
(vhdl-offsets-alist, vhdl-mode, vhdl-doc-keywords): Doc fixes.
(vhdl-template-map-init): Add vhdl-template-context.
(vhdl-mode-syntax-table): Support VHDL'08 block comments.
(vhdl-create-mode-menu): Add some entries.
(vhdl-08-keywords, vhdl-08-types, vhdl-08-attributes)
(vhdl-08-functions, vhdl-08-packages, vhdl-08-directives): New constants.
(vhdl-directives): New variable.
(vhdl-words-init, vhdl-template-process)
(vhdl-template-replace-header-keywords): Support VHDL'08.
(vhdl-abbrev-list-init): Add vhdl-directives.
(vhdl-in-comment-p, vhdl-in-literal, vhdl-win-il)
(vhdl-forward-syntactic-ws, vhdl-get-syntactic-context)
(vhdl-lineup-comment): Handle block comments and directives.
(vhdl-beginning-of-directive, vhdl-template-context)
(vhdl-template-context-hook): New functions.
(vhdl-libunit-re, vhdl-defun-re, vhdl-begin-p)
(vhdl-corresponding-begin, vhdl-get-library-unit, vhdl-regress-line)
(vhdl-align-declarations, vhdl-beginning-of-block, vhdl-end-of-block)
(vhdl-font-lock-keywords-2, vhdl-get-end-of-unit)
(vhdl-scan-context-clause): Add context.

* etc/NEWS: Mention this.