toolchain/clang-native: Set BUILD_LDFLAGS instead of LDFLAGS

BUILD_LDFLAGS is used by other recipes to emit native LDFLAGS
into their cmake toolchain files especially in QT layers. it
will usually be inherited by recipes wanting clang provide
BUILD_* variables so this is more appropriate than LDFLAGS to
pass these options.

(From OE-Core rev: f258d6e437a68bfd8a46f185cbeb17bfcccf940b)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/toolchain/clang-native.bbclass

@@ -15,4 +15,4 @@ BUILD_READELF = "${BUILD_PREFIX}llvm-readelf"
 
 DEPENDS += "clang-native libcxx-native compiler-rt-native"
 
-LDFLAGS += " --rtlib=libgcc --unwindlib=libgcc"
+BUILD_LDFLAGS += " --rtlib=libgcc --unwindlib=libgcc"