Debugger: Send flags information for cpsr register

There is a feature of target XML called flags. It allows
you to describe what a register contains.

https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html

GDB has supported this for a long time and I recently added support
in LLDB:
llvm/llvm-project@e07a421

This change adds this flags information for the cpsr register of the ARM7TDMI.
Based on the information in https://developer.arm.com/documentation/ddi0210/c/.

This is what it looks like when using GDB:
```
(gdb) info registers
r0             0x0                 0
<...>
cpsr           0x6000001f          [ Z C M=31 ]
```
And LLDB:
```
(lldb) register read cpsr
    cpsr = 0x6000001f
         = (N = 0, Z = 1, C = 1, V = 0, I = 0, F = 0, T = 0, M=31)
```

(the format is up to the debugger, lldb is a lot more verbose at the moment)

To enable this I have increased the GDB stub's outgoing buffer to 1400 bytes.
The target XML is just above 130 bytes with the flags added.

include/mgba/internal/debugger/gdb-stub.h

@@ -14,7 +14,7 @@ CXX_GUARD_START
 
 #include <mgba-util/socket.h>
 
-#define GDB_STUB_MAX_LINE 1200
+#define GDB_STUB_MAX_LINE 1400
 #define GDB_STUB_INTERVAL 32
 
 enum GDBStubAckState {

src/debugger/gdb-stub.c

@@ -50,7 +50,17 @@ static const char* TARGET_XML = "<target version=\"1.0\">"
                                 "<reg name=\"sp\" bitsize=\"32\" type=\"data_ptr\"/>"
                                 "<reg name=\"lr\" bitsize=\"32\"/>"
                                 "<reg name=\"pc\" bitsize=\"32\" type=\"code_ptr\"/>"
-                                "<reg name=\"cpsr\" bitsize=\"32\" regnum=\"25\"/>"
+                                "<flags id=\"cpsr_flags\" size=\"4\">"
+                                "<field name=\"N\" start=\"31\" end=\"31\"/>"
+                                "<field name=\"Z\" start=\"30\" end=\"30\"/>"
+                                "<field name=\"C\" start=\"29\" end=\"29\"/>"
+                                "<field name=\"V\" start=\"28\" end=\"28\"/>"
+                                "<field name=\"I\" start=\"7\" end=\"7\"/>"
+                                "<field name=\"F\" start=\"6\" end=\"6\"/>"
+                                "<field name=\"T\" start=\"5\" end=\"5\"/>"
+                                "<field name=\"M\" start=\"0\" end=\"4\"/>"
+                                "</flags>"
+                                "<reg name=\"cpsr\" bitsize=\"32\" regnum=\"25\" type=\"cpsr_flags\"/>"
                                 "</feature>"
                                 "</target>";