Fix a couple gdb.dwarf2/ testcases with "clang -flto"

Fix a couple gdb.dwarf2/ testcases with "clang -flto"

gdb.dwarf2/atomic-type.exp and gdb.dwarf2/ada-linkage-name.exp fail
when testing with "clang -flto" as compiler, like:

 $ make check TESTS="gdb.dwarf2/ada-linkage-name.exp" RUNTESTFLAGS="CC_FOR_TARGET='clang -flto'"

because with -flto, functions that aren't referenced anywhere are not
emitted in the final binary.  Fix it by adding uses of the functions.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/ada-linkage-name.c (main): Move to the bottom of the
	file and add calls to first and second.
	* gdb.dwarf2/atomic.c (main): Move to the bottom of the file and
	add call to f.