tcg/arm: Clarify tcg_out_bx for arm4 host

In theory this would re-enable usage of QEMU on an armv4 host.
Whether this is worthwhile is debatable -- we've been unconditionally
issuing the armv5t BX instruction in the prologue since 2011 without
complaint.  Possibly we should simply require an armv6 host.
Signed-off-by: NRichard Henderson <rth@twiddle.net>

tcg/arm/tcg-target.inc.c

@@ -329,11 +329,6 @@ static const uint8_t tcg_cond_to_arm_cond[] = {
     [TCG_COND_GTU] = COND_HI,
 };
 
-static inline void tcg_out_bx(TCGContext *s, int cond, int rn)
-{
-    tcg_out32(s, (cond << 28) | 0x012fff10 | rn);
-}
-
 static inline void tcg_out_b(TCGContext *s, int cond, int32_t offset)
 {
     tcg_out32(s, (cond << 28) | 0x0a000000 |
@@ -402,6 +397,18 @@ static inline void tcg_out_mov_reg(TCGContext *s, int cond, int rd, int rm)
     }
 }
 
+static inline void tcg_out_bx(TCGContext *s, int cond, TCGReg rn)
+{
+    /* Unless the C portion of QEMU is compiled as thumb, we don't
+       actually need true BX semantics; merely a branch to an address
+       held in a register.  */
+    if (use_armv5t_instructions) {
+        tcg_out32(s, (cond << 28) | 0x012fff10 | rn);
+    } else {
+        tcg_out_mov_reg(s, cond, TCG_REG_PC, rn);
+    }
+}
+
 static inline void tcg_out_dat_imm(TCGContext *s,
                 int cond, int opc, int rd, int rn, int im)
 {
@@ -977,7 +984,7 @@ static inline void tcg_out_st8(TCGContext *s, int cond,
  * with the code buffer limited to 16MB we wouldn't need the long case.
  * But we also use it for the tail-call to the qemu_ld/st helpers, which does.
  */
-static inline void tcg_out_goto(TCGContext *s, int cond, tcg_insn_unit *addr)
+static void tcg_out_goto(TCGContext *s, int cond, tcg_insn_unit *addr)
 {
     intptr_t addri = (intptr_t)addr;
     ptrdiff_t disp = tcg_pcrel_diff(s, addr);
@@ -987,15 +994,9 @@ static inline void tcg_out_goto(TCGContext *s, int cond, tcg_insn_unit *addr)
         return;
     }
 
+    assert(use_armv5t_instructions || (addri & 1) == 0);
     tcg_out_movi32(s, cond, TCG_REG_TMP, addri);
-    if (use_armv5t_instructions) {
-        tcg_out_bx(s, cond, TCG_REG_TMP);
-    } else {
-        if (addri & 1) {
-            tcg_abort();
-        }
-        tcg_out_mov_reg(s, cond, TCG_REG_PC, TCG_REG_TMP);
-    }
+    tcg_out_bx(s, cond, TCG_REG_TMP);
 }
 
 /* The call case is mostly used for helpers - so it's not unreasonable