tools/mpy_ld.py: Give better error for unsupported ARM absolute relocs.

This is a known limitation, so better to give a clear warning than a
catch-all AssertionError.  Happens for example when trying to use
soft-float on ARCH=armv6m

Also give more details on the assertion for unknown relocations, such that
one can see which symbol it affects etc, to aid in debugging.

References issue #14430.

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

tools/mpy_ld.py

@@ -702,9 +702,13 @@ def do_relocation_text(env, text_addr, r):
     elif env.arch.name == "EM_RISCV":
         (addr, value) = process_riscv32_relocation(env, text_addr, r)
 
+    elif env.arch.name == "EM_ARM" and r_info_type == R_ARM_ABS32:
+        # happens for soft-float on armv6m
+        raise ValueError("Absolute relocations not supported on ARM")
+
     else:
         # Unknown/unsupported relocation
-        assert 0, r_info_type
+        assert 0, (r_info_type, s.name, s.entry, env.arch.name)
 
     # Write relocation
     if env.arch.name == "EM_RISCV":