[llvm-objcopy][COFF] Fix section name encoding

The section name encoding for `llvm-objcopy` had two main issues, the
first is that the size used for the `snprintf` in the original code is
incorrect because `snprintf` adds a null byte, so this code was only
able to encode offsets of 6 digits - `/`, `\0` and 6 digits of the
offset - rather than the 7 digits it should support.

And the second part is that it didn't support the base64 encoding for
offsets larger than 7 digits.

This issue specifically showed up when using the `clang-offload-bundler`
with a binary containing a lot of symbols/sections, since it uses
`llvm-objcopy` to add the sections containing the offload code.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D118692