Remove mention of mipsel target_arch

The `mipsel` `target_arch` was introduced with the initial MIPSel
support (82ec1ae),
but was subsequently removed with implementation of the Flexible Target
Specification (Rust RFC 0131,
3a8f4ec).
This is the only remaining instance in rustc. All others are in the libc
repo, and are fixed in rust-lang/libc@b367659.

src/libpanic_unwind/gcc.rs

@@ -124,7 +124,7 @@ const UNWIND_DATA_REG: (i32, i32) = (0, 1); // RAX, RDX
 #[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
 const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1 / X0, X1
 
-#[cfg(any(target_arch = "mips", target_arch = "mipsel", target_arch = "mips64"))]
+#[cfg(any(target_arch = "mips", target_arch = "mips64"))]
 const UNWIND_DATA_REG: (i32, i32) = (4, 5); // A0, A1
 
 #[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))]