Don't pass -Wa,-g on clang.

clang's integrated assembler does not appear to support debug symbols
yet. (And somehow CMake has another statefulness bug that causes this to
only manifest on a second run, so the try bots aren't noticing.)

Also don't clobber the existing CMAKE_ASM_FLAGS value.

Change-Id: Ia04345f0dc2f9833ac13912d761cfab3479c10cd
Reviewed-on: https://boringssl-review.googlesource.com/11083
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

@@ -37,7 +37,10 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   endif()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${C_CXX_FLAGS} -Wmissing-prototypes -Wold-style-definition -Wstrict-prototypes")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ${C_CXX_FLAGS} -Wmissing-declarations")
-  set(CMAKE_ASM_FLAGS "-Wa,-g")
+  # Clang's integerated assembler does not support debug symbols.
+  if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} -Wa,-g")
+  endif()
 elseif(MSVC)
   set(MSVC_DISABLED_WARNINGS_LIST
       "C4100" # 'exarg' : unreferenced formal parameter