util: Add interval-tree.c

Copy and simplify the Linux kernel's interval_tree_generic.h,
instantiating for uint64_t.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderso

util: Add interval-tree.c

Copy and simplify the Linux kernel's interval_tree_generic.h,
instantiating for uint64_t.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

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