This patch consolidates the recv Linux syscall implementation on
sysdeps/unix/sysv/linux/recv.c. The changes are:
1. Remove recv from auto-generation syscalls.list on the architecture
that uses __NR_recv.
2. Define __NR_recv for architectures that supports it. It was done
instead of defining in default kernel-features.h because current Linux
practice for new ports is to implement only __NR_recvfrom [1] and it will
require adding new kernel-features for ports that do not require it
(aarch64 for instance).
3. Remove __ASSUME_RECVFROM_FOR_RECV_SYSCALL and decide to use
__NR_recvfrom for recv generation based on __ASSUME_RECVFROM_SYSCALL.
Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
aarch64-linux-gnu, arm-linux-gnueabihf, and powerpc64le-linux-gnu.