[AArch64][Driver] Allow runtime detection to override default feaures.

Currently, most extensions controlled through -march and -mcpu options
are handled in a bitset of AArch64::ExtensionSet. However, extensions
detected at runtime for native compilation are handled in a separate
list of CPU features; once most of the parsing logic has run, the bitset
is converted to a list of features, added after the features detected at
runtime, and the resulting list is used from there on.

This has the downside that runtime-detected features are unable to
override default CPU extensions. For example, if a CPU enables +aes in
its processor definition, but aes is not found at runtime, the feature
currently remains enabled---even though unsupported---because default
features are enabled after the runtime logic attempts to disable them.

This patch inserts runtime-detected features directly into the extension
set such that these options can take precedence over extensions enabled
by default. The general parsing order for mcpu=native becomes:
1. CPU defaults;
2. Runtime detection;
3. Further +featureA+nofeatureB options;
4. Other parsing decisions.

This allows features that are found to be unsupported at runtime to be
removed from the list of features supported by targets that enable them
by default.

(cherry picked from commit 9458d2a0f403f4e79130da56b74d7828867f332c)