mips-linux-user: Always support rdhwr.

The kernel will emulate this instruction if it's not supported
natively.  This insn is used for TLS, among other things, and
so is required by modern glibc.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Cc: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

target-mips/translate.c

@@ -8111,7 +8111,11 @@ gen_rdhwr (CPUMIPSState *env, DisasContext *ctx, int rt, int rd)
 {
     TCGv t0;
 
+#if !defined(CONFIG_USER_ONLY)
+    /* The Linux kernel will emulate rdhwr if it's not supported natively.
+       Therefore only check the ISA in system mode.  */
     check_insn(env, ctx, ISA_MIPS32R2);
+#endif
     t0 = tcg_temp_new();
 
     switch (rd) {