[Support] [BLAKE3] Remove .hidden directives from windows-gnu assembly sources

COFF symbols don't have anything corresponding to a `.hidden` flag;
both GNU binutils as and LLVM's built-in assembler errors out on
these directives.

This reverts one part of
7f05aa2d, fixing builds for
mingw x86_64.

Differential Revision: https://reviews.llvm.org/D122893