fix compilation with latest clang/llvm 7.0

The bcc build with latest clang/llvm 7.0 has the following
compilation error:

 /home/yhs/work/llvm/build/install/lib/libclangCodeGen.a(CodeGenAction.cpp.o):
  In function `clang::BackendConsumer::Initialize(clang::ASTContext&)':
  CodeGenAction.cpp:(.text._ZN5clang15BackendConsumer10InitializeERNS_10ASTContextE[_ZN5clang15BackendConsumer10InitializeERNS_10ASTContextE]+0xc): undefined reference to `clang::FrontendTimesIsEnabled'
 ...

The latest clang defined clang::FrontendTimesIsEnabled in
libclangFrontend.a and used in libclangCodeGen.a.

Due to the library order specified in clang_libs.cmake:
  # order is important
  set(clang_libs
    ${libclangFrontend}
    ...
    ${libclangCodeGen}
and the fact clang_libs here is used to build bcc-static,
the definition of clang::FrontendTimesIsEnabled is dropped
since nobody used it before the definition is checked by the compiler.

The fix here is to keep all the definitions/uses for clang_libs/llvm_libs
when buildint static library.

Tested on clang 4.0, 5.0, 6.0 and latest 7.0, all works fine.

Signed-off-by: Yonghong Song <yhs@fb.com>