linux: Consolidate Linux getsockopt implementation

This patch consolidates the getsockopt Linux syscall implementation on
sysdeps/unix/sysv/linux/getsockopt.c.  The changes are:

  1. Remove it from auto-generation syscalls.list on all architectures.

  2. Add __ASSUME_GETSOCKOPT_SYSCALL as default and undef if for
     specific kernel versions on some architectures.

This also fix a potential issue where 32-bit time_t ABI should use the
linux getsockopt which overrides the underlying SO_* constants used for
socket timestamping for _TIME_BITS=64.

Checked on x86_64-linux-gnu and i686-linux-gnu.