Fix mul128.h to support MSVC ARM64 target

[nursik]

I assume this is the only change required to bring ARM64 support for Windows. Build was done using VCPKG (arm64-windows triplet). Tests are OK.

[nursik]

I would like to know if there is any progress or any concerns about the change?

[codecov-commenter]

Codecov Report

Merging #3000 (c2baf41) into master (f792822) will decrease coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3000      +/-   ##
==========================================
- Coverage   92.54%   92.53%   -0.01%     
==========================================
  Files         577      577              
  Lines       66781    66781              
  Branches     6395     6394       -1     
==========================================
- Hits        61800    61796       -4     
- Misses       4948     4952       +4     
  Partials       33       33              

Impacted Files	Coverage Δ
src/lib/utils/cpuid/cpuid_x86.cpp	76.62% <0.00%> (-11.69%)	:arrow_down:
src/lib/pubkey/elgamal/elgamal.cpp	97.29% <0.00%> (+1.35%)	:arrow_up:
src/lib/asn1/der_enc.cpp	86.33% <0.00%> (+2.48%)	:arrow_up:

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[vladimir-aubrecht]

@randombit Do you think we could get this tiny change merged? 😊

[randombit]

Sorry about the delay here