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-mips.mk b/core/combo/TARGET_linux-mips.mk
index 5ec537a..92ad21a 100644
--- a/core/combo/TARGET_linux-mips.mk
+++ b/core/combo/TARGET_linux-mips.mk
@@ -121,7 +121,6 @@
 
 libc_root := bionic/libc
 libm_root := bionic/libm
-libstdc++_root := bionic/libstdc++
 
 
 ## on some hosts, the target cross-compiler is not available so do not run this command
@@ -148,7 +147,6 @@
 $(combo_2nd_arch_prefix)TARGET_C_INCLUDES := \
 	$(libc_root)/arch-mips/include \
 	$(libc_root)/include \
-	$(libstdc++_root)/include \
 	$(KERNEL_HEADERS) \
 	$(libm_root)/include \
 	$(libm_root)/include/mips \
@@ -162,7 +160,7 @@
 
 $(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