github: Drop -static-libstdc++ -static-libgcc from the openmp tests

With arm64ec now being built as arm64x, sharing DLLs with the
regular aarch64 target, we no longer need these options - the
regular libc++.dll/libunwind.dll from $PATH (from the toolchain
bin directory) work for the arm64ec build as well.

.github/workflows/build.yml

@@ -609,7 +609,6 @@ jobs:
             -DCMAKE_CXX_COMPILER=${{matrix.arch}}-w64-mingw32-clang++ `
             -DCMAKE_ASM_MASM_COMPILER=llvm-ml `
             -DLLVM_ENABLE_RUNTIMES="openmp" `
-            -DOPENMP_TEST_FLAGS="-static-libstdc++ -static-libgcc" `
             -DLIBOMP_ASMFLAGS=${{matrix.asmflag}} `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja