Fix "set cwd ..." on Cygwin, part 2

Even after the previous patch, on both native and gdbserver Cygwin, we
get:

 (gdb) set cwd /cygdrive/d/cygwin-gdb/build-testsuite/outputs/gdb.base/exitsignal
 (gdb) start
 Temporary breakpoint 3 at 0x100401094: file /home/alves/rocm/gdb/src/gdb/testsuite/gdb.base/segv.c, line 26.
 Starting program: /cygdrive/d/cygwin-gdb/build-testsuite/outputs/gdb.base/exitsignal/exitsignal.exe
 ❌️ Error creating process /cygdrive/d/cygwin-gdb/build-testsuite/outputs/gdb.base/exitsignal/exitsignal.exe (error 6): The handle is invalid.
 (gdb)

On the native side, this is because in
windows_nat_target::create_inferior, we unconditionally convert
forward slashes to backward slashes:

  /cygdrive/d/cygwin-gdb/build-testsuite/outputs/gdb.base/exitsignal
  =>
  \cygdrive\d\cygwin-gdb\build-testsuite\outputs\gdb.base\exitsignal

and then cygwin_conv_path(CCP_POSIX_TO_WIN_W) does nothing on such
path, as the backward slashes make the path not look like a Unix-style
path.

CreateProcess then fails to CD into that directory, as that's not a
real Windows native path.

The fix is to not do the slashes replacement on Cygwin.

On the GDBserver side, we're just completely missing the
cygwin_conv_path logic.  This commit adds it.  The code isn't shared
with GDB because GDB uses wide chars, and GDBserver uses narrow char.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I004f2a562757a566423f6acb9aecfcc1a7f2f746
commit-id: 85aa8c22