libc: newlibc: Fix recursive gettimeofday() calls on non-Posix systems

Calling gettimeofday() from _gettimeofday() in a non-Posix build
environment can result in a recursive call loop, causing a stack
overflow. Modify _gettimeofday() to return -1 for non-posix systems
(the previous behaviour that was added in #22508).

Fixes #41095

Signed-off-by: Binu Jacob <bjj@planetinnovation.com.au>