futils: fix undefined behavior in O_FSYNC fallback definition by cehoffman · Pull Request #7211 · libgit2/libgit2

The fallback definition of O_FSYNC uses `(1 << 31)`, which is undefined
behavior in C. The literal `1` is a signed int, and left-shifting into
the sign bit of a signed integer is undefined per the C standard.

This causes crashes on arm64 Linux with musl libc (which doesn't define
O_FSYNC), manifesting as:

    thread panic: left shift of 1 by 31 places cannot be represented
    in type 'int'

Fall back to O_SYNC when available, since it is the POSIX standard name
for the same flag. On platforms where neither is defined (e.g. Windows),
use (1 << 30) as a sentinel value that avoids the sign bit.