[gdb/testsuite] Handle with_set arch

[gdb/testsuite] Handle with_set arch

I realized that the more irregular output of show arch:
...
(gdb) show arch^M
The target architecture is set to "auto" (currently "i386").^M
...
would be a problem for something like:
...
with_set arch powerpc:common64 {}
...
and indeed:
...
(gdb) set arch powerpc:common64^M
The target architecture is set to "powerpc:common64".^M
(gdb) FAIL: gdb.base/foo.exp: set arch powerpc:common64
...
and:
...
(gdb) set arch set to "auto" (currently "i386")^M
Undefined item: "set".^M
...

Fix this in with_set by handling this type of output.

Tested on x86_64-linux.