morello-binutils: Adjust c64_valid_cap_range calculation

morello-binutils: Adjust c64_valid_cap_range calculation

This function had a buggy implementation of rounding a value up to a
given power of 2.  Aligning to a multiple of 16 would align to a
multiple of 32 and so on.

This was observable when linking object files that had very large
objects in them.  The compiler would ensure that these objects are large
enough that they are exactly representable, but the linker would
complain that they are not because the linker asserted extra alignment
than the compiler.

Here we fix the bug, add a few testcases, and adjust an existing
testcase in the area.