musl: fix SIGSTKFLT name on mips

This was incorrectly named in upstream musl and fixed in
bminor/musl@cabc369

This commit adds SIGEMT unconditionally as adding a constant should not
cause any backwards-incompatible changes.

Author: xbjfk

libc-test/build.rs

@@ -4307,6 +4307,10 @@ fn test_linux(target: &str) {
             if old_musl && name == "RLIM_NLIMITS" {
                 return true;
             }
+            // Incorrectly named and renamed upstream:
+            if old_musl && name == "SIGSTKFLT" {
+                return true;
+            }
         }
         match name {
             // These constants are not available if gnu headers have been included

src/unix/linux_like/linux/musl/b32/mips/mod.rs

@@ -563,6 +563,13 @@ pub const SIGTSTP: c_int = 24;
 pub const SIGURG: c_int = 21;
 pub const SIGIO: c_int = 22;
 pub const SIGSYS: c_int = 12;
+pub const SIGEMT: c_int = 7;
+#[cfg(not(musl_v1_2_3))]
+#[deprecated(
+    since = "0.2.173",
+    note = "This signal actually corresponds to SIGEMT was incorrectly named in musl.
+        As it does not exist on mips it will be dropped entirely in a future release"
+)]
 pub const SIGSTKFLT: c_int = 7;
 pub const SIGPOLL: c_int = crate::SIGIO;
 pub const SIGPWR: c_int = 19;