arch-arm: gdb support Thumb ISA

BaseRemoteGDB only checks if the breakpoint size equals to MachInst size.
However, there are two kinds of instruction size in ARM. We should allow
the 16-bits breakpoint for Thumb ISA.

Change-Id: I79c0e1503092ecf233c719f2e354739edb8e6b25
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52624
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/remote_gdb.cc

@@ -361,4 +361,11 @@ RemoteGDB::gdbRegs()
         return &regCache32;
 }
 
+bool
+RemoteGDB::checkBpLen(size_t len)
+{
+    // 2 for Thumb ISA, 4 for ARM ISA.
+    return len == 2 || len == 4;
+}
+
 } // namespace gem5

src/arch/arm/remote_gdb.hh

@@ -120,6 +120,7 @@ class RemoteGDB : public BaseRemoteGDB
   public:
     RemoteGDB(System *_system, int _port);
     BaseGdbRegCache *gdbRegs() override;
+    bool checkBpLen(size_t len) override;
     std::vector<std::string>
     availableFeatures() const override
     {