Re-enable FTL JIT

[mfikes]

#534 appears to no longer be reproducible with latest JavaScriptCore on macOS.

Having FTL JIT enabled again would be good for perf:

cljs.user=> (simple-benchmark [x (range 1e6)] (apply + x) 1e3)
[x (range 1000000)], (apply + x), 1000 runs, 2301 msecs

with it off:

cljs.user=> (simple-benchmark [x (range 1e6)] (apply + x) 1e3)
[x (range 1000000)], (apply + x), 1000 runs, 5426 msecs

It still occurs on Ubuntu 16.04, though:

JSC_useFTLJIT=true planck -D andare:0.7.0 -O simple -e"(require 'cljs.core.async)"
Segmentation fault (core dumped)

[mfikes]

Actually, it can still crash on macOS when compiling macros:

### AOT compiling macro namespaces
script/build: line 142: 36713 Segmentation fault: 11  planck-c/build/planck -sk planck-cljs/out/macros-tmp -e"(require 'cljs.analyzer)" -e"(do (set! cljs.analyzer/*cljs-warnings* (assoc cljs.analyzer/*cljs-warnings* :undeclared-var false)) nil)" -e "(require-macros 'planck.repl 'planck.core 'planck.shell 'planck.from.io.aviso.ansi 'clojure.template 'cljs.spec.alpha 'cljs.spec.test.alpha 'cljs.spec.gen.alpha 'cljs.test 'cljs.pprint)"
Build Failed.

[mfikes]

One way to reproduce the issue is to run the tests: script/test will exhibit a SIGSEGV pretty easily with JSC_useFTLJIT=true:

$ script/test
Running unit tests...
script/test: line 8: 63933 Segmentation fault: 11  script/test-unit

[mfikes]

By running the test in #534 it looks like we are also good on Linux

JSC_useFTLJIT=true plk -Sdeps '{:deps {andare {:mvn/version "0.7.0"}}}' --optimizations simple -e"(require 'cljs.core.async)"