math/big: fix Int.Divide remainder calculation when r aliases y (fix #80882)

[SparshGarg999]

Fixes #80882

Context & Problem

In math/big, calling Int.Divide(x, y, y, mode) evaluates quotient q into the destination receiver before computing remainder r. When remainder destination r aliases divisor y, mutating y/r during quotient evaluation causes the subsequent remainder calculation r = x - y*q to evaluate using an already mutated divisor, producing incorrect remainder values.

Solution & Changes

• Updated Int.Divide in src/math/big/int.go to handle remainder receiver aliasing with the divisor parameter by staging remainder computations before mutating aliased quotient/divisor targets.

Testing & Verification

• Added test cases in src/math/big/int_test.go verifying ceiling, floor, and truncation division when remainder destination aliases the divisor (q, r := q.Divide(x, y, y, big.Ceil)).

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