Fix register mapping to avoid volatile registers and issues with R12

Signed-off-by: Alan Jowett <alan.jowett@microsoft.com>

vm/ubpf_jit_x86_64.c

@@ -52,15 +52,21 @@ static int platform_parameter_registers[] = {
     RCX, RDX, R8, R9
 };
 #define RCX_ALT R15
+// Register assignments:
+// BPF R0-R4 are "volatile"
+// BPF R5-R10 are "non-volatile"
+// Map BPF volatile registers to x64 volatile and map BPF non-volatile to
+// x64 non-volatile.
+// Avoid R12 as we don't support encoding modrm modifier for using R12.
 static int register_map[REGISTER_MAP_SIZE] = {
     RAX,
     R15,
     RDX,
     R8,
     R9,
-    R10,
-    R11,
-    R12,
+    RBX,
+    RDI,
+    RSI,
     R13,
     R14,
     RBP,