Don't pass error from syscallenter() to syscallret().

syscallret() doesn't use error anymore. Fix a few other places to permit
removing the return value from syscallenter() entirely.

• Remove a duplicated assertion from arm's syscall().
• Use td_errno for amd64_syscall_ret_flush_l1d.

Reviewed by: kib
MFC after: 1 month
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D2090