Use GCC 9.2.0 instead of 9.5.0

Because of unsoundness found in GCC 9.3+.

Author: Kobzol

src/ci/docker/host-aarch64/dist-aarch64-linux/Dockerfile

@@ -46,9 +46,11 @@ WORKDIR /tmp
 RUN mkdir /home/user
 COPY scripts/shared.sh /tmp/
 
-# Need at least GCC 5.1 to compile LLVM
+# Need at least GCC 9 to compile LLVM
+# GCC 9.3+ have an unsoundness problem (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189), so we
+# stay at 9.2.0.
 COPY scripts/build-gcc.sh /tmp/
-ENV GCC_VERSION=9.5.0
+ENV GCC_VERSION=9.2.0
 ENV GCC_BUILD_TARGET=aarch64-unknown-linux-gnu
 RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 

src/ci/docker/host-x86_64/dist-i686-linux/Dockerfile

@@ -49,9 +49,11 @@ WORKDIR /tmp
 RUN mkdir /home/user
 COPY scripts/shared.sh /tmp/
 
-# Need at least GCC 5.1 to compile LLVM nowadays
+# Need at least GCC 9 to compile LLVM
+# GCC 9.3+ have an unsoundness problem (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189), so we
+# stay at 9.2.0.
 COPY scripts/build-gcc.sh /tmp/
-ENV GCC_VERSION=9.5.0
+ENV GCC_VERSION=9.2.0
 ENV GCC_BUILD_TARGET=i686-pc-linux-gnu
 RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -52,9 +52,11 @@ WORKDIR /tmp
 RUN mkdir /home/user
 COPY scripts/shared.sh /tmp/
 
-# Need at least GCC 5.1 to compile LLVM nowadays
+# Need at least GCC 9 to compile LLVM
+# GCC 9.3+ have an unsoundness problem (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189), so we
+# stay at 9.2.0.
 COPY scripts/build-gcc.sh /tmp/
-ENV GCC_VERSION=9.5.0
+ENV GCC_VERSION=9.2.0
 ENV GCC_BUILD_TARGET=x86_64-pc-linux-gnu
 RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 

src/ci/docker/scripts/build-gcc.sh

@@ -7,7 +7,7 @@ source shared.sh
 # This version is specified in the Dockerfile
 GCC=$GCC_VERSION
 
-curl https://ftp.gnu.org/gnu/gcc/gcc-$GCC/gcc-$GCC.tar.xz | xzcat | tar xf -
+curl https://ci-mirrors.rust-lang.org/rustc/gcc/gcc-$GCC.tar.xz | xzcat | tar xf -
 cd gcc-$GCC
 
 # FIXME(#49246): Remove the `sed` below.