Only enable AArch64 extensions if the compiler supports them

Only enable AArch64 extensions if the compiler supports them

We already have some logic in the configure.sh file to selectively
disable code dependent on particular architecture extensions, however we
do not yet have anything to check that the compiler being supplied
recognises and can compile code using these extensions.

This commit adds compiler "-march=..." flag tests to the existing
extension-disable loop so that we now correctly disable extensions that
are not supported by the compiler. For AArch64 this loop also needs to
move below the existing compiler/OS handling to ensure that prefixes
like $CROSS are handled correctly before running compiler tests.

NOTE: Fix build error in aarch64.

Bug: webm:1841
Change-Id: I936b911c4b0ebf03abc34b7532b2bb4568129f57
(cherry picked from commit fa50b26848ebd89915ec2a5a138a23f1fe69d5eb)