Hello @andrey-podko, in general, pulling Proton apart and building individual components separately isn't supported.
Having written that, unknown use of instruction mnemonic without a size suffix is the line of interest from the build log.
proton 10.0x1 2026-05proton 11.0x1 2026-05
wine-vanilla-11.9 - fine with Clang-21
wine-staging-11.9 - fine with Clang-21
wine-proton-11.0.1_beta2 fails with Clang-21 but fine with GCC-16.1
wine-proton-11.0.1_beta3 fails with Clang-21but fine with GCC-16.1
wine-proton-10.0.4 - fine with Clang-21
build.log
Probably this one commit breaks Clang build...
https://github.com/ValveSoftware/wine/commit/96666ae3beb901d37076cdf07a7efc5ce7907b18#diff-16c324b2569b540398c661095dfb2c2b241c99921750bf1a23b6ff8620317bafR2071-R2072