x86inc: Prefer r14/r15 over r12/r13 on x86-64

Due to a peculiarity in the ModR/M addressing encoding, the r12 and r13
registers sometimes requires an additional byte when used as a base register.

r14 and r15 doesn't have that issue, so prefer using them.

libavutil/x86/x86inc.asm

@@ -426,10 +426,10 @@ DECLARE_REG 7,  rdi, 64
 DECLARE_REG 8,  rsi, 72
 DECLARE_REG 9,  rbx, 80
 DECLARE_REG 10, rbp, 88
-DECLARE_REG 11, R12, 96
-DECLARE_REG 12, R13, 104
-DECLARE_REG 13, R14, 112
-DECLARE_REG 14, R15, 120
+DECLARE_REG 11, R14, 96
+DECLARE_REG 12, R15, 104
+DECLARE_REG 13, R12, 112
+DECLARE_REG 14, R13, 120
 
 %macro PROLOGUE 2-5+ 0 ; #args, #regs, #xmm_regs, [stack_size,] arg_names...
     %assign num_args %1
@@ -530,10 +530,10 @@ DECLARE_REG 7,  R10, 16
 DECLARE_REG 8,  R11, 24
 DECLARE_REG 9,  rbx, 32
 DECLARE_REG 10, rbp, 40
-DECLARE_REG 11, R12, 48
-DECLARE_REG 12, R13, 56
-DECLARE_REG 13, R14, 64
-DECLARE_REG 14, R15, 72
+DECLARE_REG 11, R14, 48
+DECLARE_REG 12, R15, 56
+DECLARE_REG 13, R12, 64
+DECLARE_REG 14, R13, 72
 
 %macro PROLOGUE 2-5+ ; #args, #regs, #xmm_regs, [stack_size,] arg_names...
     %assign num_args %1