base: fdReady(): Fix timeouts > ~49 days overflowing. Fixes #14262.

On 64-bit UNIX and Windows, Haskell `Int` has 64 bits
but C `int msecs` has 32 bits, resulting in an overflow.

This commit fixes it by switching fdReady() to take int64_t,
into which a Haskell `Int` will always fit.

(Note we could not switch to `long long` because that is
32 bit on 64-bit Windows machines.)

Further, to be able to actually wait longer than ~49 days,
we put loops around the waiting syscalls (they all accept only
32-bit integers).

Note the timer signal would typically interrupt the syscalls
before the ~49 days are over, but you can run Haskell programs
without the timer signal, an we want it to be correct in all
cases.

Reviewers: bgamari, austin, hvr, NicolasT, Phyx

Reviewed By: bgamari, Phyx

Subscribers: syd, Phyx, rwbarton, thomie

GHC Trac Issues: #14262

Differential Revision: https://phabricator.haskell.org/D4011

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