perf tests clang: Fix function name for clang IR test

As stated in tests/llvm-src-base.c, the name of the bpf function should
be "bpf_func__SyS_epoll_pwait" but this clang test fails as it tries to
lookup "bpf_func__SyS_epoll_wait".

Before applying patch:

55: builtin clang support                                 :
55.1: builtin clang compile C source to IR                : FAILED!
55.2: builtin clang compile C source to ELF object        : Skip

After applying patch:

55: builtin clang support                                 :
55.1: builtin clang compile C source to IR                : Ok
55.2: builtin clang compile C source to ELF object        : Ok

Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Fixes: e67d52d4

 ("perf clang: Update test case to use real BPF script")
Link: http://lkml.kernel.org/r/20180404180419.19056-3-sandipan@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>