Allow ';' as a directory separator

Hi,
I find two fails in source-dir.exp on mingw32 host.

(gdb) directory /nOtExStInG/a /nOtExStInG/b /nOtExStInG/c^M
Warning: /nOtExStInG/a: No such file or directory.^M
Warning: /nOtExStInG/b: No such file or directory.^M
Warning: /nOtExStInG/c: No such file or directory.^M
Source directories searched: /nOtExStInG/a;/nOtExStInG/b;/nOtExStInG/c;$cdir;$cwd^M
(gdb) FAIL: gdb.base/source-dir.exp: directory /nOtExStInG/a /nOtExStInG/b /nOtExStInG/c
directory /nOtExStInG/b /nOtExStInG/d /nOtExStInG/c^M
Warning: /nOtExStInG/b: No such file or directory.^M
Warning: /nOtExStInG/d: No such file or directory.^M
Warning: /nOtExStInG/c: No such file or directory.^M
Source directories searched: /nOtExStInG/b;/nOtExStInG/d;/nOtExStInG/c;/nOtExStInG/a;$cdir;$cwd^M
(gdb) FAIL: gdb.base/source-dir.exp: directory /nOtExStInG/b /nOtExStInG/d /nOtExStInG/c

The regular expression expects ':' and ';' is path separator on Windows.
This patch is to allow ';' as a path separator in regular expression.

This patch is similar to Dan's patch to fix a similar problem here

 Re: directory separators on minGW hosts
 https://sourceware.org/ml/gdb-patches/2006-02/msg00359.html

It is obvious.  Pushed it in.

gdb/testsuite:

2014-03-31  Yao Qi  <yao@codesourcery.com>

	* gdb.base/source-dir.exp: Allow ';' as a directory separator.