Fix dynamic type resolution for LOC_CONST and LOC_CONST_BYTES symbols

PR rust/31005 points out that dynamic type resolution of a LOC_CONST
or LOC_CONST_BYTES symbol will fail, leading to output like:

    from_index=<error reading variable: Cannot access memory at address 0x0>

This patch fixes the problem by using the constant value or bytes when
performing type resolution.

Thanks to tpzker@thepuzzlemaker.info for a first version of this
patch.

I also tested this on a big-endian PPC system (cfarm203).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31005