tcg/i386: Always use %ebp for TCG_AREG0

For x86_64, this can remove a REX prefix resulting in smaller code
when manipulating globals of type i32, as we move them between backing
store via cpu_env, aka TCG_AREG0.
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

tcg/i386/tcg-target.h

@@ -84,6 +84,8 @@ typedef enum {
     TCG_REG_RBP = TCG_REG_EBP,
     TCG_REG_RSI = TCG_REG_ESI,
     TCG_REG_RDI = TCG_REG_EDI,
+
+    TCG_AREG0 = TCG_REG_EBP,
 } TCGReg;
 
 /* used for function call generation */
@@ -194,12 +196,6 @@ extern bool have_avx2;
 #define TCG_TARGET_extract_i64_valid(ofs, len) \
     (((ofs) == 8 && (len) == 8) || ((ofs) + (len)) == 32)
 
-#if TCG_TARGET_REG_BITS == 64
-# define TCG_AREG0 TCG_REG_R14
-#else
-# define TCG_AREG0 TCG_REG_EBP
-#endif
-
 static inline void flush_icache_range(uintptr_t start, uintptr_t stop)
 {
 }