MI: Print frame architecture when printing frames on an MI channel

MI: Print frame architecture when printing frames on an MI channel

When printing frames on an MI channel also print the frame
architecture like in:

    (gdb)
    -stack-list-frames 3 3
    ^done,stack=
    [frame={level="3",addr="0x000107a4",func="foo",
      file="recursive2.c",fullname="/home/foo/bar/recursive2.c",
      line="14",arch="i386:x86_64"}]
   (gdb)

This is useful for MI clients that need to know the architecture in
order to perform further analysis, for example to use their own
disassembler to analyze machine code.

gdb/Changelog:
2018-08-22  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* stack.c (print_frame): Print frame architecture when printing on
        an MI output.
	* NEWS: Mention new "arch" attribute in frame output.

gdb/testsuite/Changelog
2018-08-22  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* lib/mi-support.exp (mi_expect_stop): Update regexp to
	accommodate new "arch" field in frame output.
	* gdb.mi/mi-return.exp: Likewise.
	* gdb.mi/mi-stack.exp: Likewise.
	* gdb.mi/mi-syn-frame.exp: Likewise.
	* gdb.mi/user-selected-context-sync.exp: Likewise.

gdb/doc/Changelog
2018-08-22  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* gdb.texinfo (The -stack-list-frames Command): Update description
	to mention "arch".
	Update MI examples throughout the document to contain "arch" in
	frame output.