JIT

Do not set target.llvm_version on empty string #20

Job	Run time
Interpreter (Debug)	10m 3s
i686-pc-windows-msvc/msvc (Release)	15m 58s
aarch64-pc-windows-msvc/msvc (Debug)	22m 52s
x86_64-pc-windows-msvc/msvc (Debug)	24m 56s
i686-pc-windows-msvc/msvc (Debug)	24m 9s
x86_64-pc-windows-msvc/msvc (Release)	14m 15s
aarch64-pc-windows-msvc/msvc (Release)	18m 10s
x86_64-apple-darwin/clang (Release)	13m 59s
aarch64-apple-darwin/clang (Debug)	12m 12s
x86_64-apple-darwin/clang (Debug)	17m 34s
x86_64-unknown-linux-gnu/gcc (Release)	6m 56s
x86_64-unknown-linux-gnu/gcc (Debug)	12m 8s
aarch64-unknown-linux-gnu/gcc (Debug)	7m 9s
aarch64-apple-darwin/clang (Release)	9m 17s
aarch64-unknown-linux-gnu/gcc (Release)	5m 32s
	3h 35m 10s

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