Recognize _ in attribute names

Ada attribute names can contain "_", but the lexer currently does not
allow this -- even though the "attributes" array lists some attributes
spelled this way.

This patch fixes the bug and adds test cases for the existing
attributes.

This was reviewed off-list by Joel.  I'm checking it in.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* ada-lex.l: Allow "_" in attribute names.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/formatted_ref.exp (test_p_x_addr): Check
	'unchecked_access and 'unrestricted_access as well.