Correct casts for ARM in machine_vectors.h (#2246) #3182

Job	Run time
GCC build regression check (no tests)	1m 5s
Ubuntu GCC, Code Coverage (x10)	29m 19s
Ubuntu GCC with NTL (assert, x2)	7m 58s
MSVC (x1)	26m 23s
MinGW GCC (x0.5)	10m 40s
Ubuntu Clang, BLAS and examples (x5)	13m 32s
Nemo.jl	18m 46s
Ubuntu ARM (x5)	12m 4s
macOS-M1, Clang (x3)	8m 10s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 18s
	2h 22m 15s

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