Fix an issue with lld and llvm-ar 3.9 vs genlib

Dockerfile

@@ -94,10 +94,15 @@ ENV AS=llvm-as
 ENV NM=llvm-nm
 
 # Build mingw with our freshly built cross compiler
+# Since somewhere between llvm 3.8 and 3.9, SVN rev 273373, git mirror commit
+# d319cd64a4a15,, llvm-ar tries to detect the format (gnu vs bsd) of the
+# existing .a file. For files generated by genlib, it seems to detect the
+# wrong format, leading to lld later segfaulting when trying to link.
+# Force the flag -format gnu to llvm-ar in this step to work around this issue.
 RUN cd mingw-w64/mingw-w64-crt && \
     autoreconf -vif && \
     mkdir build && cd build && \
-    ../configure --host=$TARGET_TUPLE --prefix=$MINGW_PREFIX \
+    AR="llvm-ar -format gnu" ../configure --host=$TARGET_TUPLE --prefix=$MINGW_PREFIX \
         --disable-lib32 --disable-lib64 --enable-libarm32 \
         --with-genlib && \
     make -j4 && \