Fix uclibc LLVM target triples

`uclibc` is not an environment understood by LLVM, it is only a concept
in Clang that can be selected with `-muclibc` (it affects which dynamic
linker is passed to the static linker's `-dynamic-linker` flag).

In fact, using `uclibcgnueabi`/`uclibc` is actively harmful, as it
prevents LLVM from seeing that the target is gnu-like; we should use
`gnueabi`/`gnu` directly instead.

compiler/rustc_target/src/spec/targets/armv5te_unknown_linux_uclibceabi.rs

@@ -2,7 +2,7 @@ use crate::spec::{FloatAbi, Target, TargetMetadata, TargetOptions, base};
 
 pub(crate) fn target() -> Target {
     Target {
-        llvm_target: "armv5te-unknown-linux-uclibcgnueabi".into(),
+        llvm_target: "armv5te-unknown-linux-gnueabi".into(),
         metadata: TargetMetadata {
             description: Some("Armv5TE Linux with uClibc".into()),
             tier: Some(3),

compiler/rustc_target/src/spec/targets/mips_unknown_linux_uclibc.rs

@@ -4,7 +4,7 @@ use crate::spec::{Target, TargetMetadata, TargetOptions, base};
 
 pub(crate) fn target() -> Target {
     Target {
-        llvm_target: "mips-unknown-linux-uclibc".into(),
+        llvm_target: "mips-unknown-linux-gnu".into(),
         metadata: TargetMetadata {
             description: Some("MIPS Linux with uClibc".into()),
             tier: Some(3),

compiler/rustc_target/src/spec/targets/mipsel_unknown_linux_uclibc.rs

@@ -2,7 +2,7 @@ use crate::spec::{Target, TargetMetadata, TargetOptions, base};
 
 pub(crate) fn target() -> Target {
     Target {
-        llvm_target: "mipsel-unknown-linux-uclibc".into(),
+        llvm_target: "mipsel-unknown-linux-gnu".into(),
         metadata: TargetMetadata {
             description: Some("MIPS (LE) Linux with uClibc".into()),
             tier: Some(3),

compiler/rustc_target/src/spec/targets/x86_64_unknown_l4re_uclibc.rs

@@ -8,7 +8,7 @@ pub(crate) fn target() -> Target {
     base.panic_strategy = PanicStrategy::Abort;
 
     Target {
-        llvm_target: "x86_64-unknown-l4re-uclibc".into(),
+        llvm_target: "x86_64-unknown-l4re-gnu".into(),
         metadata: TargetMetadata {
             description: None,
             tier: Some(3),