rustc_back/target: remove musl_base

it's the same as linux_musl_base

src/librustc_back/target/arm_unknown_linux_musleabi.rs

@@ -11,7 +11,7 @@
 use target::Target;
 
 pub fn target() -> Target {
-    let mut base = super::musl_base::opts();
+    let mut base = super::linux_musl_base::opts();
 
     // Most of these settings are copied from the arm_unknown_linux_gnueabi
     // target.

src/librustc_back/target/arm_unknown_linux_musleabihf.rs

@@ -11,7 +11,7 @@
 use target::Target;
 
 pub fn target() -> Target {
-    let mut base = super::musl_base::opts();
+    let mut base = super::linux_musl_base::opts();
 
     // Most of these settings are copied from the arm_unknown_linux_gnueabihf
     // target.

src/librustc_back/target/armv7_unknown_linux_musleabihf.rs

@@ -11,7 +11,7 @@
 use target::Target;
 
 pub fn target() -> Target {
-    let mut base = super::musl_base::opts();
+    let mut base = super::linux_musl_base::opts();
 
     // Most of these settings are copied from the armv7_unknown_linux_gnueabihf
     // target.

src/librustc_back/target/mod.rs

@@ -59,7 +59,6 @@ mod freebsd_base;
 mod linux_base;
 mod linux_musl_base;
 mod openbsd_base;
-mod musl_base;
 mod netbsd_base;
 mod solaris_base;
 mod windows_base;