file: Avoid strange istringstream crash in cmake.org binaries on Alpine Linux

Somehow using `istringstream` and `operator >>` to parse an integer
crashes on Alpine Linux, at least when compiled with the settings we use
for the official `cmake.org` Linux binaries.  Since commit fd0c285b12
(file: Fix types of the OFFSET and LIMIT arguments, 2022-01-04,
v3.23.0-rc1~133^2), this causes the `file(READ)` command to crash when
parsing its `LIMIT` or `OFFSET` argument.  Parse the input string with
our dedicated helper to avoid the crash.

Fixes: #23872