[gsymutil] Fix a warning on systems with 32-bit `off_t` (#189524)
The size of `off_t` isn't specified, so it can be either 32 or 64 bits
depending on the system. In particular, on LLP64 systems like windows
it's generally only 32 bits. This means the `if (StrtabSize >
UINT32_MAX)` check added in #181458 may warn on such systems (Giving
-Wsign-compare).
Given that `FileWriter::tell` (and the underlying `raw_ostream::tell`)
explicitly return `uint64_t`, the simplest fix is to just use the return
type of the function instead of potentially truncating. Since the same
logic applies even where we don't happen to have a warning here, I've
applied this for all of these uses of `off_t`.