RFR[xs]: 8245137: aarch64 ICache flush depends on enabling gnu extensions

[Kim Barrett]

Please review this small change to the aarch64 implementation of
ICache::invalidate_word ICache::invalidate_range.  Rather than calling
__clear_cache directly, they are changed to use __builtin___clear_cache.
The direct call to __clear_cache requires either providing a
declaration for that function or enabling gnu extensions.  Using the
builtin requires neither.

CR:
https://bugs.openjdk.java.net/browse/JDK-8245137

Webrev:
https://cr.openjdk.java.net/~kbarrett/8245137/open.00/

Testing:
mach5 tier1-5 with -std=c++14 as part of experimental C++14 support.
mach5 tier1

Note that there are unrelated build failures with -std=c++98.  Among
other things, we use va_copy in shared code, which is not available
until C++11 (because it's a C99 feature), but is made available as a
gnu extension with -std=gnu++98.