[arm][testsuite] Fix -march tests in effective target checks auto-generation

[arm][testsuite] Fix -march tests in effective target checks auto-generation

There is a typo in the armv8.1-a and armv8.2-a effective target check generators.
They are not actually used anywhere in the testsuite as far as I can tell, but the fix is obvious.

     * lib/target-supports.exp: Fix -march arguments in arm arch effective
     target check autogenerator for armv8.1-a and armv8.2-a.

From-SVN: r256799