Update code for MSVC intrinsics.
Update code for MSVC intrinsics.

MSVC provides the `_umul128` and `__shiftright128` intrinsics
for x64 only (not x86, ARM, ARM64), so this code should be guarded
with `_M_X64`.

Work around LLVM#37755 "Implement __shiftright128 for
x86_64-pc-windows-msvc".

Nothing inspects the macro `USE_MANUAL_64_BIT_MUL`.

MSVC doesn't need `#pragma intrinsic` for this;
including intrin.h is sufficient. (Verified by inspecting codegen.)

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu