gdbserial: ensure registers are loaded in SetDX/SetSP/SetPC (#2054)

These methods only work if registers have been loaded once after the
last resume, there's probably no code path that calls SetXX before
Thread.Registers but lets make sure it can't happen anyway.

pkg/proc/gdbserial/gdbserver.go

@@ -1840,6 +1840,7 @@ func (r *gdbRegisters) FloatLoadError() error {
 
 // SetPC will set the value of the PC register to the given value.
 func (t *gdbThread) SetPC(pc uint64) error {
+	_, _ = t.Registers() // Registes must be loaded first
 	t.regs.setPC(pc)
 	if t.p.gcmdok {
 		return t.p.conn.writeRegisters(t.strID, t.regs.buf)
@@ -1850,6 +1851,7 @@ func (t *gdbThread) SetPC(pc uint64) error {
 
 // SetSP will set the value of the SP register to the given value.
 func (t *gdbThread) SetSP(sp uint64) error {
+	_, _ = t.Registers() // Registes must be loaded first
 	t.regs.setSP(sp)
 	if t.p.gcmdok {
 		return t.p.conn.writeRegisters(t.strID, t.regs.buf)
@@ -1860,6 +1862,7 @@ func (t *gdbThread) SetSP(sp uint64) error {
 
 // SetDX will set the value of the DX register to the given value.
 func (t *gdbThread) SetDX(dx uint64) error {
+	_, _ = t.Registers() // Registes must be loaded first
 	t.regs.setDX(dx)
 	if t.p.gcmdok {
 		return t.p.conn.writeRegisters(t.strID, t.regs.buf)