MIPS: run tst-mode-switch-{1,2,3}.c using test-skeleton.c

MIPS: run tst-mode-switch-{1,2,3}.c using test-skeleton.c

For some reasons I have not investigated yet, tst-mode-switch-1 hangs on
a MIPS UTM-8 machine running an o32 userland and a 3.6.1 kernel.

This patch changes the test so that it runs under the test-skeleton
framework, causing the test to fail after a timeout instead of hanging
the whole testsuite. At the same time, also change the tst-mode-switch-2
and tst-mode-switch-3 tests.

Changelog:
	* sysdeps/mips/tst-mode-switch-1.c (main): Converted to ...
	(do_test): ... this.
	(TEST_FUNCTION): New macro.
	 Include test-skeleton.c.
	* sysdeps/mips/tst-mode-switch-2.c (main): Likewise.
	* sysdeps/mips/tst-mode-switch-3.c (main): Likewise.