mpy_ld.py: Support modules larger than 4KiB on armv6m

Using the BL instruction in ARM Thumb
All other architectures seems to support larger modules already

Fixes exception such as:

    File "micropython/tools/mpy_ld.py", line 904, in build_mpy
      jump = env.arch.asm_jump(entry_offset)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "micropython/tools/mpy_ld.py", line 92, in asm_jump_thumb
      assert b_off >> 11 == 0 or b_off >> 11 == -1, b_off
                               ^^^^^^^^^^^^^^^^^
    AssertionError: 6153

Signed-off-by: Jon Nordby <jononor@gmail.com>

tools/mpy_ld.py

@@ -89,8 +89,17 @@ def asm_jump_x86(entry):
 def asm_jump_thumb(entry):
     # Only signed values that fit in 12 bits are supported
     b_off = entry - 4
-    assert b_off >> 11 == 0 or b_off >> 11 == -1, b_off
-    return struct.pack("<H", 0xE000 | (b_off >> 1 & 0x07FF))
+    if b_off >> 11 == 0 or b_off >> 11 == -1:
+        return struct.pack("<H", 0xE000 | (b_off >> 1 & 0x07FF))
+    else:
+        # Use BL instruction
+        # Two 16-bit instructions in sequence
+        # each on form: 1111 HOOO OOOO OOOO
+        # first instruction has H=0, and has high 11 bits of the Offset
+        # second instruction has H=1, and has low 11 bits of the Offset
+        b0 = 0xF000 | (b_off >> 12 & 0x07FF)
+        b1 = 0xF800 | (b_off >> 1 & 0x7FF)
+        return struct.pack("<HH", b0, b1)
 
 
 def asm_jump_thumb2(entry):