This patch consolidates the accept Linux syscall implementation on
sysdeps/unix/sysv/linux/accept.c. The changes are:
1. Remove accept from auto-generation syscalls.list on the architecture
that uses __NR_accept.
2. Define __NR_accept as default (__ASSUME_ACCEPT_SYSCALL) and undef for
architectures that do not support it.
3. Remove __ASSUME_ACCEPT4_FOR_ACCEPT_SYSCALL and decide to use
__NR_accept4 for accept generation based on __ASSUME_ACCEPT4_SYSCALL.
Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
aarch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.