Rollup merge of #39647 - japaric:sparc, r=alexcrichton

build std for sparc64-unknown-linux-gnu

r? @alexcrichton

panicking / unwinding is broken (#39646) but with std available at least people
will be able to debug that issue on real hardware

Author: frewsxcv

src/ci/docker/cross/Dockerfile

@@ -14,6 +14,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   zlib1g-dev \
   g++-arm-linux-gnueabi \
   g++-arm-linux-gnueabihf \
+  gcc-sparc64-linux-gnu \
+  libc6-dev-sparc64-cross \
   bzip2 \
   patch
 
@@ -60,9 +62,11 @@ ENV TARGETS=$TARGETS,mipsel-unknown-linux-musl
 ENV TARGETS=$TARGETS,arm-unknown-linux-musleabi
 ENV TARGETS=$TARGETS,arm-unknown-linux-musleabihf
 ENV TARGETS=$TARGETS,armv7-unknown-linux-musleabihf
+ENV TARGETS=$TARGETS,sparc64-unknown-linux-gnu
 
 ENV CC_mipsel_unknown_linux_musl=mipsel-openwrt-linux-gcc \
-    CC_mips_unknown_linux_musl=mips-openwrt-linux-gcc
+    CC_mips_unknown_linux_musl=mips-openwrt-linux-gcc \
+    CC_sparc64_unknown_linux_gnu=sparc64-linux-gnu-gcc
 
 # Suppress some warnings in the openwrt toolchains we downloaded
 ENV STAGING_DIR=/tmp

src/tools/build-manifest/src/main.rs

@@ -78,6 +78,7 @@ static TARGETS: &'static [&'static str] = &[
     "powerpc64-unknown-linux-gnu",
     "powerpc64le-unknown-linux-gnu",
     "s390x-unknown-linux-gnu",
+    "sparc64-unknown-linux-gnu",
     "wasm32-unknown-emscripten",
     "x86_64-apple-darwin",
     "x86_64-apple-ios",