gdb/linux-tdep: move "Perms" column right

gdb/linux-tdep: move "Perms" column right

Commit 29ef4c0699e1 ("gdb/linux-tdep.c: Add Perms to the 'info proc
mappings' output") has broken test gdb.base/info-proc.exp on Linux,
because it changes the output of "info proc mappings" in a way that the
test does not expect (my bad for not testing before pushing).

I looked at how FreeBSD handles this, since I remembered it did show
permission flags.  It looks like this:

          Start Addr           End Addr       Size     Offset   Flags   File
            0x200000           0x243000    0x43000        0x0  r-- CN-- /usr/local/bin/tmux

(I think that `Flags` and the flags not being aligned is not
intentional)

The test passes on FreeBSD, because the test looks for four hex numbers
in a row and ignores the rest:

    ".*Mapped address spaces:.*${hex}${ws}${hex}${ws}${hex}${ws}${hex}.*"

I suggest fixing it on Linux by moving the flags column to the same
place as in the FreeBSD output.  It makes things a bit more consistent
between OSes, and we don't have to touch the test.

At the same time, make use of the actual length of the permission's
string to specify the number of characters to print.

Before this patch, the output looks like:

          Start Addr           End Addr   Perms       Size     Offset objfile
      0x55dd4b544000     0x55dd4b546000   r--p      0x2000        0x0 /usr/bin/sleep

and after, it looks like:

          Start Addr           End Addr       Size     Offset  Perms  objfile
      0x5622ae662000     0x5622ae664000     0x2000        0x0  r--p   /usr/bin/sleep

Change-Id: If0fc167b010b25f97a3c54e2f491df4973ccde8f