Support sparc-unknown-linux-gnu again

Currently, a patch is needed due to "undefined reference to `__sync_val_compare_and_swap_8'" error
(due to max_atomic_width is wrongly set to 64:
https://github.com/rust-lang/rust/blob/1.70.0/compiler/rustc_target/src/spec/sparc_unknown_linux_gnu.rs#L8)

README.md

@@ -23,11 +23,10 @@
 
 | libc | GCC | clang | C++ | test | host |
 | ---- | --- | ----- | --- | ---- | ---- |
-| glibc [1] | [1] | host [3] | ✓ (libstdc++) | ✓ (qemu) [2] | [1] |
+| glibc [1] | [1] | host | ✓ (libstdc++) | ✓ (qemu) [2] | [1] |
 
 [1] See target list below for details<br>
 [2] Except for powerpc-unknown-linux-gnuspe, riscv32gc-unknown-linux-gnu, and x86_64-unknown-linux-gnux32<br>
-[3] Except for sparc-unknown-linux-gnu<br>
 
 ([Dockerfile](docker/linux-gnu.Dockerfile))
 
@@ -61,6 +60,7 @@
 | `riscv64gc-unknown-linux-gnu` | 2.27 | 7.4.0 | x86_64/aarch64 linux (glibc 2.27+) |
 | `s390x-unknown-linux-gnu` | 2.27 | 7.4.0 | x86_64/aarch64 linux (glibc 2.27+) |
 | `sparc64-unknown-linux-gnu` | 2.27 | 7.4.0 | x86_64 linux (glibc 2.27+) |
+| `sparc-unknown-linux-gnu` (tier3) | 2.27 | 7.4.0 | x86_64 linux (glibc 2.27+) |
 | `thumbv7neon-unknown-linux-gnueabihf` | 2.27 | 7.4.0 | x86_64/aarch64 linux (glibc 2.27+) |
 | `x86_64-unknown-linux-gnu` | host (x86_64 host) / 2.27 (aarch64 host) | host (x86_64 host) / 7.4.0 (aarch64 host) | x86_64/aarch64 linux (glibc 2.27+) |
 | `x86_64-unknown-linux-gnux32` | 2.27 | 7.4.0 | x86_64/aarch64 linux (glibc 2.27+) |

docker/test-base/patches/std+sparc-unknown-linux-gnu.diff

@@ -0,0 +1,13 @@
+diff --git a/src/thread/mod.rs b/src/thread/mod.rs
+index e4581c2..4ff9d03 100644
+--- a/src/thread/mod.rs
++++ b/src/thread/mod.rs
+@@ -1099,7 +1099,7 @@ fn exhausted() -> ! {
+         }
+
+         cfg_if::cfg_if! {
+-            if #[cfg(target_has_atomic = "64")] {
++            if #[cfg(all(target_has_atomic = "64", not(target_arch = "sparc")))] {
+                 use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
+
+                 static COUNTER: AtomicU64 = AtomicU64::new(0);

docker/test/test.sh

@@ -210,11 +210,10 @@ case "${RUST_TARGET}" in
 esac
 no_cc_bin=""
 case "${RUST_TARGET}" in
-    # TODO(clang,sparc-unknown-linux-gnu): clang: error: unknown argument: '-mv8plus'
     # TODO(clang,linux-uclibc): interpreter should be /lib/ld-uClibc.so.0
     # TODO(clang,linux-uclibc): qemu: uncaught target signal 11 (Segmentation fault) - core dumped
     # TODO(clang,mips-musl-sf): interpreter should be /lib/ld-musl-mips(el)-sf.so.1
-    sparc-unknown-linux-gnu | mips-unknown-linux-musl | mipsel-unknown-linux-musl | *-linux-uclibc*)
+    mips-unknown-linux-musl | mipsel-unknown-linux-musl | *-linux-uclibc*)
         case "${cc}" in
             clang) no_cc_bin=1 ;;
         esac
@@ -250,11 +249,10 @@ esac
 # Whether or not to build the test.
 no_build_test=""
 case "${RUST_TARGET}" in
-    # TODO(sparc-unknown-linux-gnu): undefined reference to `__sync_val_compare_and_swap_8'
     # TODO(sparc64-unknown-openbsd):
     #     /sparc64-unknown-openbsd/bin/sparc64-unknown-openbsd7.0-ld: /sparc64-unknown-openbsd/sparc64-unknown-openbsd/usr/lib/libm.a(s_fmin.o): in function `*_libm_fmin':
     #         /usr/src/lib/libm/src/s_fmin.c:35: undefined reference to `__isnan'
-    sparc-unknown-linux-gnu | sparc64-unknown-openbsd) no_build_test=1 ;;
+    sparc64-unknown-openbsd) no_build_test=1 ;;
 esac
 # Whether or not to run the test.
 no_run_test=""

platform-support-status-tier3.md

@@ -106,7 +106,7 @@ See [platform-support-status.md](platform-support-status.md) for Tier 1 & Tier 2
 - [ ] riscv64gc-unknown-netbsd
 - [x] riscv64gc-unknown-openbsd
 - [x] s390x-unknown-linux-musl
-- [ ] sparc-unknown-linux-gnu
+- [x] sparc-unknown-linux-gnu
 - [x] sparc64-unknown-netbsd
 - [x] sparc64-unknown-openbsd
 - [ ] thumbv4t-none-eabi

tools/target-list-shared.sh

@@ -35,7 +35,7 @@ linux_gnu_targets=(
     riscv32gc-unknown-linux-gnu # tier3
     riscv64gc-unknown-linux-gnu
     s390x-unknown-linux-gnu
-    # sparc-unknown-linux-gnu # tier3
+    sparc-unknown-linux-gnu # tier3
     sparc64-unknown-linux-gnu
     thumbv7neon-unknown-linux-gnueabihf
     x86_64-unknown-linux-gnu