Remove mention of mipsel target_arch

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

Author: xen0n

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"))]