testsuite: use pwd -W to convert from Unix to Windows paths

When on a MinGW host, standard_output_file uses a regular expression to
convert Unix-style paths of the form "/c/foo" to "c:/foo".  This is
needed because the paths we pass to GDB (for example, with the "file"
command) need to be in the Windows form.

However, the regexp only works if your binutils-gdb repo is under a
`/[a-z]/...` path (the Unix paths mapping to Windows drives).
Presumably, that works if you clone the repo in Windows, then access it
through `/c/...`.

In my case, I've cloned the repository directly inside my MinGW shell,
so in /home/smarchi.  The regexp therefore doesn't work for me.  The
path doesn't get transformed, and the file command fails when running
any test:

    (gdb) file /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.arch/i386-bp_permanent/i386-bp_permanent
    /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.arch/i386-bp_permanent/i386-bp_permanent: No such file or directory.

A safer way to do this is to execute `pwd -W` while in the directory we
want the path for, this is what this patch does.

I have also considered using the using the cygpath utility to do the
conversion.  It can be used to convert any MinGW path into its Windows
equivalent.  Despite originally coming from Cygwin, the cygpath utility
is distributed by MinGW-w64 and can be used in that environment.
However, it's not distributed with the non-MinGW-w64 MinGW.

The `pwd -W` trick only works with directories that exist, which is the
case here, so it's sufficient.

With this, the file command in the test succeeds:

    (gdb) file C:/msys64/home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.arch/i386-bp_permanent/i386-bp_permanent
    Reading symbols from C:/msys64/home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.arch/i386-bp_permanent/i386-bp_permanent...

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (standard_output_file): Use `pwd -W` to convert
	from Unix to Windows path.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-11  Simon Marchi  <simon.marchi@efficios.com>
+
+	* lib/gdb.exp (standard_output_file): Use `pwd -W` to convert
+	from Unix to Windows path.
+
 2020-03-11  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.ada/minsyms.exp: Set language to ada.

gdb/testsuite/lib/gdb.exp

@@ -4899,7 +4899,7 @@ proc standard_output_file {basename} {
     file mkdir $dir
     # If running on MinGW, replace /c/foo with c:/foo
     if { [ishost *-*-mingw*] } {
-        set dir [regsub {^/([a-z])/} $dir {\1:/}]
+        set dir [exec sh -c "cd ${dir} && pwd -W"]
     }
     return [file join $dir $basename]
 }