Revert "Docker.system-clang: Fix building with older LLVM versions after recent changes"

This reverts commit 490dfff.

This is no longer necessary, as we now can build runtimes using
the latest LLVM 19 versions, that support building sanitizers
for all architectures.

Author: mstorsjo

Dockerfile.system-clang

@@ -22,10 +22,7 @@ WORKDIR /build
 
 ENV TOOLCHAIN_PREFIX=/opt/llvm-mingw
 
-# build-compiler-rt.sh assumes that sanitizers can be built for all architectures.
-# Sanitizers can be built for aarch64 since LLVM 14 and for armv7 since LLVM 19.
-# For simplicity, just skip building for armv7 when building an older version.
-ARG TOOLCHAIN_ARCHS="i686 x86_64 aarch64"
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 ARG DEFAULT_CRT=ucrt