[wwwdocs] Buildstat update for 7.x

[Tom G. Christensen]

Latest results for 7.x.

-tgc

Testresults for 7.4.0:
  x86_64-w64-mingw32

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--- /home/tgc/projects/gcc/wwwdocs/htdocs/gcc-7/buildstat.html	2019-02-27 18:33:53.559979051 +0100
+++ /tmp/tmp.fa3s1NLqiV	2019-02-27 21:38:18.452766358 +0100
@@ -126,6 +126,7 @@
 <td>x86_64-w64-mingw32</td>
 <td> </td>
 <td>Test results:
+<a href="https://gcc.gnu.org/ml/gcc-testresults/2019-01/msg00222.html">7.4.0</a>,
 <a href="https://gcc.gnu.org/ml/gcc-testresults/2018-02/msg00308.html">7.3.0</a>,
 <a href="https://gcc.gnu.org/ml/gcc-testresults/2017-08/msg01364.html">7.2.0</a>,
 <a href="https://gcc.gnu.org/ml/gcc-testresults/2017-05/msg00410.html">7.1.0</a>