Update ABISysV_arm64::RegisterIsVolatile to accept registers prefixed with r

[xiaobai]

Summary

While the specification says that the 64bit registers are prefixed with
x, it seems that many people still use r. Until recently, we had been using
the r prefix instead of the x prefix in ds2. This caused lldb to fail during
unwinding. I think it's reasonable to check for a register prefixed with r,
since some people still choose to use r.

[sas]

Looks good to me, but I'd like to see what @clayborg thinks about this.

[xiaobai]

@clayborg: Any objection to this?

[clayborg]

Back from vacation, sorry for the delay.

source/Plugins/ABI/SysV-arm64/ABISysV_arm64.cpp

if (name[0] == 'p' && name[1] == 'c') // pc
       return false;
     if (name[0] == 'f' && name[1] == 'p') // fp
       return false;
     if (name[0] == 's' && name[1] == 'p') // sp
       return false;
     if (name[0] == 'l' && name[1] == 'r') // lr
       return false;
 
-    if (name[0] == 'x') {
+    if (name[0] == 'x' || name[0] == 'r') {
       // Volatile registers: x0-x18
       // Although documentation says only x19-28 + sp are callee saved
       // We ll also have to treat x30 as non-volatile.
       // Each dwarf frame has its own value of lr.
       // Return false for the non-volatile gpr regs, true for everything else
       switch (name[1]) {
       case '1':
         switch (name[2]) {