Make boringssl_gtest_main a STATIC library

Prior to 3.12 (which we won't be requiring until July), OBJECT libraries
cannot be used with target_link_libraries. That means they cannot pick
up INTERFACE_INCLUDE_DIRECTORIES, which makes them pretty unusable in
the "modern CMake" style.

Just switch it to a static library to unbreak the build in CMake 3.10.

For some link ordering reason I don't understand, this also requires
explicitly linking boringssl_gtest to libcxx when we USE_CUSTOM_LIBCXX
is set.

Change-Id: Ia9d8351551f5da060248aa3ca73fe04473bf62aa
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/57345
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

ssl/CMakeLists.txt

diff --git a/ssl/CMakeLists.txt b/ssl/CMakeLists.txt
index 147ab58..d8d997e 100644
--- a/ssl/CMakeLists.txt
+++ b/ssl/CMakeLists.txt
@@ -52,8 +52,6 @@
   span_test.cc
   ssl_test.cc
   ssl_c_test.c
-
-  $<TARGET_OBJECTS:boringssl_gtest_main>
 )
-target_link_libraries(ssl_test test_support_lib boringssl_gtest ssl crypto)
+target_link_libraries(ssl_test test_support_lib boringssl_gtest_main ssl crypto)
 add_dependencies(all_tests ssl_test)