modpost: detect section mismatch for R_ARM_THM_{MOVW_ABS_NC,MOVT_ABS}

modpost: detect section mismatch for R_ARM_THM_{MOVW_ABS_NC,MOVT_ABS}

When CONFIG_THUMB2_KERNEL is enabled, modpost fails to detect some
types of section mismatches.

  [test code]

    #include <linux/init.h>

    int __initdata foo;
    int get_foo(void) { return foo; }

It is apparently a bad reference, but modpost does not report anything.

The test code above produces the following relocations.

  Relocation section '.rel.text' at offset 0x1e8 contains 2 entries:
   Offset     Info    Type            Sym.Value  Sym. Name
  00000000  0000052f R_ARM_THM_MOVW_AB 00000000   .LANCHOR0
  00000004  00000530 R_ARM_THM_MOVT_AB 00000000   .LANCHOR0

Currently, R_ARM_THM_MOVW_ABS_NC and R_ARM_THM_MOVT_ABS are just skipped.

Add code to handle them. I checked arch/arm/kernel/module.c to learn
how the offset is encoded in the instruction.

One more thing to note for Thumb instructions - the st_value is an odd
value, so you need to mask the bit 0 to get the offset. Otherwise, you
will get an off-by-one error in the nearest symbol look-up.

It is documented in "ELF for the ARM Architecture" [1]:

  In addition to the normal rules for symbol values the following rules
  shall also apply to symbols of type STT_FUNC:

   * If the symbol addresses an Arm instruction, its value is the
     address of the instruction (in a relocatable object, the offset
     of the instruction from the start of the section containing it).

   * If the symbol addresses a Thumb instruction, its value is the
     address of the instruction with bit zero set (in a relocatable
     object, the section offset with bit zero set).

   * For the purposes of relocation the value used shall be the address
     of the instruction (st_value & ~1).

[1]: https://github.com/ARM-software/abi-aa/blob/main/aaelf32/aaelf32.rst

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>