testsuite: Save dg-do-what-default in mve.exp

testsuite: Save dg-do-what-default in mve.exp

mve.exp changed the default dg-do action to "assemble", but then
left it like that for later exp files.  This meant that in a
two-multilib test run, the first arm.exp run would have a default
of "dg-do compile" and the second would have a default of
"dg-do assemble".

2020-04-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* g++.target/arm/mve.exp: Restore the original dg-do-what-default
	before finishing.