Move selection of C++ STL into the build system.

Preparing for migration from stlport to libc++. STL selection is done
with LOCAL_CXX_STL (valid values are default, none, libc++,
libc++_static, stlport, stlport_static, bionic).

The selection of the STL is as follows:

    if LOCAL_CXX_STL == 'default'
      ifdef LOCAL_SDK_VERSION
        Use whatever STL the other NDK options have selected.
      else
        Use bionic's libstdc++ for target, GNU libstdc++ for host. This
        is compatible with the existing build options.
      endif
    else
      if LOCAL_CXX_STL == 'stlport'
        Use stlport.
      else if LOCAL_CXX_STL == 'libc++'
        Use libc++.
      else if LOCAL_CXX_STL == ''
        Don't use any STL.
      endif
    endif

Bug: 15193147
Change-Id: If712ba0ae7908d8147a69e29da5c453a183d6540
diff --git a/core/combo/TARGET_linux-x86.mk b/core/combo/TARGET_linux-x86.mk
index c17a8ef..6e5f5ac 100644
--- a/core/combo/TARGET_linux-x86.mk
+++ b/core/combo/TARGET_linux-x86.mk
@@ -70,7 +70,6 @@
 
 libc_root := bionic/libc
 libm_root := bionic/libm
-libstdc++_root := bionic/libstdc++
 
 KERNEL_HEADERS_COMMON := $(libc_root)/kernel/uapi
 KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/uapi/asm-x86 # x86 covers both x86 and x86_64.
@@ -130,7 +129,6 @@
 $(combo_2nd_arch_prefix)TARGET_C_INCLUDES := \
 	$(libc_root)/arch-x86/include \
 	$(libc_root)/include \
-	$(libstdc++_root)/include \
 	$(KERNEL_HEADERS) \
 	$(libm_root)/include \
 	$(libm_root)/include/i387 \
@@ -144,7 +142,7 @@
 
 $(combo_2nd_arch_prefix)TARGET_STRIP_MODULE:=true
 
-$(combo_2nd_arch_prefix)TARGET_DEFAULT_SYSTEM_SHARED_LIBRARIES := libc libstdc++ libm
+$(combo_2nd_arch_prefix)TARGET_DEFAULT_SYSTEM_SHARED_LIBRARIES := libc libm
 
 $(combo_2nd_arch_prefix)TARGET_CUSTOM_LD_COMMAND := true
 define $(combo_2nd_arch_prefix)transform-o-to-shared-lib-inner