ANDROID: build.config: set NDK_TRIPLE

In order to support CONFIG_UAPI_HEADER_TEST=y with a bionic sysroot
using prebuilt bionic from the NDK, we need to set a different target
triple for USERCFLAGS than what's used when cross compiling the kernel
(so that the correct headers and libc.{a|so} are found+used).

Bug: 190019968
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: Ib3e60c41b862cda9f79ff8d2c812aaa8bfb571af
diff --git a/build.config.x86_64 b/build.config.x86_64
index 8b0633e..b5ac82b 100644
--- a/build.config.x86_64
+++ b/build.config.x86_64
@@ -12,3 +12,5 @@
 modules.builtin
 modules.builtin.modinfo
 "
+
+NDK_TRIPLE=x86_64-linux-android31