On architectures that don't use a syscall wrapper, sys_* function names
are set as an alias of __se_sys_* functions. Due to this, there is no
BTF associated with sys_* function names. This results in some of the
test progs failing to load. Set the SYS_PREFIX to "__se_" to fix this
issue.

Fixes: 38261f36 ("selftests/bpf: Fix probe_user test failure with clang build kernel")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/013d632aacd3e41290445c0025db6a7055ec6e18.1643973917.git.naveen.n.rao@linux.vnet.ibm.com