Although SwiftShader was building properly on the build bots, it
wasn't exporting its symbols properly. I compared the flags used in
the BUILD.gn files and in the CMakeLists.txt file and attempted to
get both build systems to use the same flags and options.

Change-Id: Idc73d4ad5b45311881e88720bf611e25110dcc70
Reviewed-on: https://swiftshader-review.googlesource.com/8576Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>