iOS and OSX: Upgrade containers to use latest osxcross and Xcode 12.0.1

Provides SDKs for iOS 14.0 and macOS 10.15.

Mono is patched to fix a regression in Xcode 11.4+, still unfixed by Apple.
Another patch is backported from Unity's Mono fork to fix another issue with
the macOS 11.0 beta SDK.

Fixes missing arguments in Xcode and iOS dockerfiles, seems like some changes
were not properly committed during the last refactor.

Dockerfile.osx

@@ -8,8 +8,8 @@ RUN if [ -z "${mono_version}" ]; then echo -e "\n\nargument mono-version is mand
       automake autoconf bzip2-devel clang libicu-devel libtool libxml2-devel llvm-devel openssl-devel yasm && \
     git clone --progress https://github.com/tpoechtrager/osxcross.git && \
     cd /root/osxcross && \
-    git checkout 542acc2ef6c21aeb3f109c03748b1015a71fed63 && \
-    ln -s /root/files/MacOSX10.14.sdk.tar.xz /root/osxcross/tarballs && \
+    git checkout 45d2f7ec2ee68c9ad51d590869609b5b3b028605 && \
+    ln -s /root/files/MacOSX10.15.sdk.tar.xz /root/osxcross/tarballs && \
     UNATTENDED=1 ./build.sh
 
 ENV OSXCROSS_ROOT=/root/osxcross
@@ -17,11 +17,13 @@ ENV PATH="/root/osxcross/target/bin:${PATH}"
 
 RUN cp -a /root/files/${mono_version} /root && \
     cd /root/${mono_version} && \
-    export CMAKE=/root/osxcross/target/bin/x86_64-apple-darwin18-cmake && \
+    patch -p1 < /root/files/patches/mono-xcode-allow-weak-import.patch && \
+    patch -p1 < /root/files/patches/mono-sys-uio-fix-preadv-misdetection.patch && \
+    export CMAKE=/root/osxcross/target/bin/x86_64-apple-darwin19-cmake && \
     NOCONFIGURE=1 ./autogen.sh && \
     ./configure --prefix=/root/dependencies/mono \
         --build=x86_64-linux-gnu \
-        --host=x86_64-apple-darwin18 \
+        --host=x86_64-apple-darwin19 \
         --disable-boehm \
         --disable-mcs-build \
         --with-tls=pthread \