Don't look for libunwind if cross-compiling.

pkg-config gets confused and doesn't know to look in, say,
/usr/lib/i386-linux-gnu when building for 32-bit. Fortunately, CMake
sets a CMAKE_CROSSCOMPILING variable whenever CMAKE_SYSTEM_NAME is set
manually (as done in util/32-bit-toolchain.cmake).

Change-Id: I638b4d54ea92ade4b2b5baa40a3c5e8c17914d46
Reviewed-on: https://boringssl-review.googlesource.com/c/34305
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2bc7feb..1dbb643 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,7 +39,7 @@
   find_program(GO_EXECUTABLE go)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND NOT CMAKE_CROSSCOMPILING)
   find_package(PkgConfig QUIET)
   if (PkgConfig_FOUND)
     pkg_check_modules(LIBUNWIND libunwind-generic)