hppa: fix loading of global pointer in _start [BZ #20277]

The patched change fixes a regression for executables compiled with the
-p option and linked with gcrt1.o.  The executables crash on startup.

This regression was introduced in 2.22 and was noticed in the gcc testsuite.

(cherry picked from commit 9765ffa7)
(cherry picked from commit 0903610f)