Previously we did this only on Darwin due to #17414. However, even on
other platforms >2GB writes are on shaky ground. POSIX explicitly says
that the result is implementation-specified and Linux will write at most
0x7ffff000, even on 64-bit platforms. Moreover, getting the sign
of the syscall result correct is tricky, as demonstrated by the fact
that T17414 currently fails on FreeBSD.

For simplicity we now just uniformly clamp to 0x7ffff000 on all
platforms.