patch: bump to 2.7.6, switch to tar.xz, add gcc2 patch. (#3200)

The 2nd patch in patch-2.7.6-gcc2.patchset is almost identical to
the 2nd one in tar-1.30-gcc2.patchset, and the files they patch:

*    tar-1.30/gnu/backupfile.c *    tar-1.30/gnu/parse-datetime.y
* patch-2.7.6/lib/backupfile.c * patch-2.7.6/lib/parse-datetime.y

are identical before and after the patch. There is only one diff,
in line 3: we have a "2017" for tar and a "2018" for patch.

sys-devel/patch/patches/patch-2.7.6.patchset

@@ -0,0 +1,23 @@
+From d3c585d16030c1bd44183499ffa4660ae7e69ec1 Mon Sep 17 00:00:00 2001
+From: Jerome Duval <jerome.duval@gmail.com>
+Date: Mon, 16 Feb 2015 17:22:43 +0000
+Subject: fstatat is in sys/stat.h
+
+
+diff --git a/lib/symlinkat.c b/lib/symlinkat.c
+index 7a663af..b6b09e2 100644
+--- a/lib/symlinkat.c
++++ b/lib/symlinkat.c
+@@ -18,6 +18,9 @@
+ 
+ #include <config.h>
+ 
++#ifdef __HAIKU__
++#include <sys/stat.h>
++#endif
+ #include <unistd.h>
+ #include <errno.h>
+ 
+-- 
+2.19.1
+