Patch Clang bug in __builtin_clzl by pushing volatile #1630

Job	Run time
GCC build regression check (no tests)	1m 10s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 13s
MSVC (x1)	9m 26s
macOS-M1, Clang (x3)	8m 20s
MinGW GCC (x0.5)	10m 28s
Nemo.jl	18m 17s
Ubuntu GCC with NTL (assert, x2)	8m 41s
Ubuntu GCC, Code Coverage (x10)	30m 49s
Ubuntu Clang, BLAS and examples (x5)	3m 55s
	1h 45m 19s

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