perf build: Allow explicitely disabling the NO_SYSCALL_TABLE variable

perf build: Allow explicitely disabling the NO_SYSCALL_TABLE variable

This is useful to see if, on x86, the legacy libaudit still works, as it
is used in architectures that don't have the SYSCALL_TABLE logic and we
want to have it tested in 'make -C tools/perf/ build-test'.

E.g.:

Without having audit-libs-devel installed:

  $ make NO_SYSCALL_TABLE=1 O=/tmp/build/perf -C tools/perf install-bin
  make: Entering directory '/home/acme/git/perf/tools/perf'
    BUILD:   Doing 'make -j12' parallel build
  <SNIP>
  Auto-detecting system features:
  <SNIP>
  ...                      libaudit: [ OFF ]
  ...                        libbfd: [ on  ]
  ...                        libcap: [ on  ]
  <SNIP>
  Makefile.config:664: No libaudit.h found, disables 'trace' tool, please install audit-libs-devel or libaudit-dev
  <SNIP>

After installing it:

  $ rm -rf /tmp/build/perf ; mkdir -p /tmp/build/perf
  $ time make NO_SYSCALL_TABLE=1 O=/tmp/build/perf  -C tools/perf install-bin ; perf test python
  make: Entering directory '/home/acme/git/perf/tools/perf'
    BUILD:   Doing 'make -j12' parallel build
    HOSTCC   /tmp/build/perf/fixdep.o
    HOSTLD   /tmp/build/perf/fixdep-in.o
    LINK     /tmp/build/perf/fixdep
  Warning: Kernel ABI header at 'tools/arch/x86/include/asm/msr-index.h' differs from latest version at 'arch/x86/include/asm/msr-index.h'
  diff -u tools/arch/x86/include/asm/msr-index.h arch/x86/include/asm/msr-index.h
  Warning: Kernel ABI header at 'tools/perf/util/hashmap.h' differs from latest version at 'tools/lib/bpf/hashmap.h'
  diff -u tools/perf/util/hashmap.h tools/lib/bpf/hashmap.h
  Warning: Kernel ABI header at 'tools/perf/util/hashmap.c' differs from latest version at 'tools/lib/bpf/hashmap.c'
  diff -u tools/perf/util/hashmap.c tools/lib/bpf/hashmap.c

  Auto-detecting system features:
  <SNIP>
  ...                      libaudit: [ on  ]
  ...                        libbfd: [ on  ]
  ...                        libcap: [ on  ]
  <SNIP>
  $ ldd ~/bin/perf | grep audit
  	libaudit.so.1 => /lib64/libaudit.so.1 (0x00007fc18978e000)
  $

Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20200529155552.463-3-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>