Auto merge of rust-lang#88952 - skrap:add-armv7-uclibc, r=nagisa

Add new tier-3 target: armv7-unknown-linux-uclibceabihf

This change adds a new tier-3 target: armv7-unknown-linux-uclibceabihf

This target is primarily used in embedded linux devices where system resources are slim and glibc is deemed too heavyweight.  Cross compilation C toolchains are available [here](https://toolchains.bootlin.com/) or via [buildroot](https://buildroot.org).

The change is based largely on a previous PR rust-lang#79380 with a few minor modifications.  The author of that PR was unable to push the PR forward, and graciously allowed me to take it over.

Per the [target tier 3 policy](https://github.com/rust-lang/rfcs/blob/master/text/2803-target-tier-policy.md), I volunteer to be the "target maintainer".

This is my first PR to Rust itself, so I apologize if I've missed things!

Author: bors

std/src/sys/unix/mod.rs

@@ -307,6 +307,9 @@ cfg_if::cfg_if! {
         #[link(name = "zircon")]
         #[link(name = "fdio")]
         extern "C" {}
+    } else if #[cfg(all(target_os = "linux", target_env = "uclibc"))] {
+        #[link(name = "dl")]
+        extern "C" {}
     }
 }
 

std/src/sys/unix/thread.rs

@@ -594,7 +594,8 @@ pub mod guard {
                 Some(stackaddr - guardsize..stackaddr)
             } else if cfg!(all(target_os = "linux", target_env = "musl")) {
                 Some(stackaddr - guardsize..stackaddr)
-            } else if cfg!(all(target_os = "linux", target_env = "gnu")) {
+            } else if cfg!(all(target_os = "linux", any(target_env = "gnu", target_env = "uclibc")))
+            {
                 // glibc used to include the guard area within the stack, as noted in the BUGS
                 // section of `man pthread_attr_getguardsize`.  This has been corrected starting
                 // with glibc 2.27, and in some distro backports, so the guard is now placed at the

unwind/src/lib.rs

@@ -63,7 +63,7 @@ cfg_if::cfg_if! {
 // don't want to duplicate it here.
 #[cfg(all(
     target_os = "linux",
-    target_env = "gnu",
+    any(target_env = "gnu", target_env = "uclibc"),
     not(feature = "llvm-libunwind"),
     not(feature = "system-llvm-libunwind")
 ))]
@@ -72,7 +72,7 @@ extern "C" {}
 
 #[cfg(all(
     target_os = "linux",
-    target_env = "gnu",
+    any(target_env = "gnu", target_env = "uclibc"),
     not(feature = "llvm-libunwind"),
     feature = "system-llvm-libunwind"
 ))]