[lld][RISCV] Handle relaxation reductions of more than 65536 bytes

In a real-world case with functions that have many, many
R_RISCV_CALL_PLT relocations due to asan and ubsan
instrumentation, all these can be relaxed by an instruction and
the net result is more than 65536 bytes of reduction in the
output .text section that totals about 1.2MiB in final size.

This changes InputSection to use a 32-bit field for bytesDropped.
The RISCV relaxation keeps track in a 64-bit field and detects
32-bit overflow as it previously detected 16-bit overflow. It
doesn't seem likely that 32-bit overflow will arise, but it's not
inconceivable and it's cheap enough to detect it.

This unfortunately increases the size of InputSection on 64-bit
hosts by a word, but that seems hard to avoid.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D150722