Enable FTL JIT by default on AArch64

Enable FTL JIT by default on AArch64
https://bugs.webkit.org/show_bug.cgi?id=144039

Reviewed by Yusuke Suzuki.

.:

* Source/cmake/OptionsWPE.cmake:
* Source/cmake/WebKitFeatures.cmake: Set FTL as enabled by default
when targeting either WTF_CPU_X86_64 or WTF_CPU_ARM64.

Tools:

* Scripts/build-jsc: Enable the FTL JIT option by default on ARM64.
* Scripts/run-jsc-stress-tests: Unmark the ARM64 and Linux combination
as an unsupported FTL platform.
* Scripts/webkitdirs.pm:
(isARM64): Returns 1 if the determined architecture is 'arm64'.
* Scripts/webkitperl/FeatureList.pm: Enable FTL_JIT and WEBASSEMBLY on ARM64.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@216575 268f45cc-cd09-0410-ab3c-d52691b4dbfc