linux: Add execveat system call wrapper

It operates similar to execve and it is is already used to implement
fexecve without requiring /proc to be mounted.  However, different
than fexecve, if the syscall is not supported by the kernel an error
is returned instead of trying a fallback.

Checked on x86_64-linux-gnu and powerpc64le-linux-gnu.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>