[formatv][FmtAlign] Use fill count of type size_t instead of uint32_t (#78459)

FmtAlign::fill() accepts a uint32_t variable while the usages operate on
size_t values. On some platform / compiler combinations, this ends up
being a narrowing conversion. Fix this by changing the function's signature.
This was first seen on MSVC x86.

Co-authored-by: Orest Chura <orest.chura@intel.com>