perf test llvm: Avoid error when PROFILE_ALL_BRANCHES is set

The 'if' keyword is a define that expands to complex code when
CONFIG_PROFILE_ALL_BRANCHES is selected, which causes a 'perf test LLVM'
failure like:

  $ ./perf test LLVM
  35: LLVM search and compile                    :
  35.1: Basic BPF llvm compile                    : Ok
  35.2: kbuild searching                          : Ok
  35.3: Compile source for BPF prologue generation: FAILED!
  35.4: Compile source for BPF relocation         : Skip

The only affected test case is bpf-script-test-prologue.c
because it uses kernel headers and has 'if' inside.

This patch undefines 'if' to make it passes perf test.

More detailed analysis from a message in this thread, also by Wang:

The problem is caused by following relocation information:

  $ readelf -a ./llvmsubtest3
  ...
     [ 5] _ftrace_branch    PROGBITS         0000000000000000  00000260
          00000000000000a0  0000000000000000  WA       0     0     4
  ...
  Relocation section '.relfunc=null_lseek file->f_mode offset orig' at
  offset 0x490 contains 4 entries:
     Offset          Info           Type           Sym. Value    Sym. Name
  000000000038  000b00000001 unrecognized: 1       0000000000000000 _ftrace_branch
  0000000000b0  000b00000001 unrecognized: 1       0000000000000000 _ftrace_branch
  000000000128  000b00000001 unrecognized: 1       0000000000000000 _ftrace_branch
  0000000001c0  000b00000001 unrecognized: 1       0000000000000000 _ftrace_branch

  Relocation section '.rel_ftrace_branch' at offset 0x4d0 contains 8 entries:
     Offset          Info           Type           Sym. Value    Sym. Name
  000000000000  000200000001 unrecognized: 1       0000000000000000 .L__func__.bpf_func__n
  000000000008  000100000001 unrecognized: 1       0000000000000015 .L.str
  000000000028  000200000001 unrecognized: 1       0000000000000000 .L__func__.bpf_func__n
  000000000030  000100000001 unrecognized: 1       0000000000000015 .L.str
  000000000050  000200000001 unrecognized: 1       0000000000000000 .L__func__.bpf_func__n
  000000000058  000100000001 unrecognized: 1       0000000000000015 .L.str
  000000000078  000200000001 unrecognized: 1       0000000000000000 .L__func__.bpf_func__n
  000000000080  000100000001 unrecognized: 1       0000000000000015 .L.str
  ...

So I think the failure is because you enabled CONFIG_PROFILE_ALL_BRANCHES.

I can reproduce your buggy result by selecting
CONFIG_PROFILE_ALL_BRANCHES in my kbuild:

  $ ./perf test LLVM
  35: LLVM search and compile                    :
  35.1: Basic BPF llvm compile                    : Ok
  35.2: kbuild searching                          : Ok
  35.3: Compile source for BPF prologue generation: FAILED!
  35.4: Compile source for BPF relocation         : Skip

Simply undef CONFIG_PROFILE_ALL_BRANCHES in clang opts not working
because it is introduced by "#include <uapi/linux/fs.h>", which override
cmdline options. So I think the best way is to undefine 'if' inside BPF
script.
Reported-and-Tested-by: NThomas-Mich Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: NWang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Zefan Li <lizefan@huawei.com>
Link: http://lkml.kernel.org/r/20170620183203.2517-1-wangnan0@huawei.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/bpf-script-test-prologue.c

@@ -10,6 +10,15 @@
 
 #include <uapi/linux/fs.h>
 
+/*
+ * If CONFIG_PROFILE_ALL_BRANCHES is selected,
+ * 'if' is redefined after include kernel header.
+ * Recover 'if' for BPF object code.
+ */
+#ifdef if
+# undef if
+#endif
+
 #define FMODE_READ		0x1
 #define FMODE_WRITE		0x2