gdb/testsuite: enable gdb.base/sect-cmd.exp test for all targets

During review of the next patch (which changes the 'section' command),
a bug was pointed out.  I wondered why no tests spotted this bug and I
found that the 'section' command test (sect-cmd.exp) is only run on
hppa targets!

In this commit I have given this test script a bit of a spring clean,
bringing it up to date with current testsuite style.  I have made some
of the patterns a little more robust, but in general my intention was
not to change the underlying meaning of any of these tests.

gdb/testsuite/ChangeLog:

	* gdb.base/sect-cmd.exp: Rewrite using modern testsuite
	techniques.  Enable the test for all targets.