_BitScanForward64 intrinsic is not available on x86

This function

https://github.com/janestreet/base/blob/aee79da69b1dd93870cbae78794b2679dfba3dfc/src/int_math_stubs.c#L25

is not supported by 32-bit MSVC (see https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64). So I get a link error.

Start at src/int_math_stubs.c line 25 and inspect the _BitScanForward64 use in a 32-bit MSVC build. Reproduce the reported link error on x86, then verify that the compatible implementation builds and links while preserving the existing integer-scan behavior.

Written by the indexing model from the issue text.