perf unwind: Fix compile error for static cross build

perf unwind: Fix compile error for static cross build

Build failure for static cross-compiling on aarch64, with libunwind-x86
provided:

  $ file ./libunwind_for_x86_on_aarch64/lib/libunwind-x86.so.8.0.1

  libunwind-x86.so.8.0.1: ELF 64-bit LSB shared object, ARM aarch64,
  version 1 (SYSV), dynamically linked, not stripped

  $ make LDFLAGS=-static LIBUNWIND_DIR=./libunwind_for_x86_on_aarch64
  ARCH=aarch64 CROSS_COMPILE=aarch64-buildroot-linux-gnu-

  ~/libperf.a(libperf-in.o): In function `find_proc_info':
  :(.text+0xae4ac): undefined reference to `_Ux86_dwarf_search_unwind_table'
  ~/libperf.a(libperf-in.o): In function `_unwind__prepare_access':
  :(.text+0xaedd0): undefined reference to `_Ux86_create_addr_space'
  :(.text+0xaee24): undefined reference to `_Ux86_set_caching_policy'
  ~/libperf.a(libperf-in.o): In function `_unwind__flush_access':
  :(.text+0xaee98): undefined reference to `_Ux86_flush_cache'
  ~/libperf.a(libperf-in.o): In function `_unwind__finish_access':
  :(.text+0xaef08): undefined reference to `_Ux86_destroy_addr_space'
  ~/libperf.a(libperf-in.o): In function `get_entries':
  :(.text+0xaf148): undefined reference to `_Ux86_init_remote'
  :(.text+0xaf184): undefined reference to `_Ux86_get_reg'
  :(.text+0xaf1a4): undefined reference to `_Ux86_step'
  collect2: error: ld returned 1 exit status
  Makefile.perf:350: recipe for target '~/perf' failed
  make[1]: *** [~/perf] Error 1
  Makefile:68: recipe for target 'all' failed
  make: *** [all] Error 2

This is because the remote libunwind library detected is not appended to
EXTLIBS variable, which will be included between 'start-group' and
'end-group' when linking.

The existing variable LIBUNWIND_LIBS is assigned to libs for local
unwind, this patch introduces a new variable EXTLIBS_LIBUNWIND for
storing remote libunwind libraries instead.

Signed-off-by: He Kuang <hekuang@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1465988636-81502-1-git-send-email-hekuang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>