toolchain/clang-native: Set BUILD_LDFLAGS instead of LDFLAGS

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.

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>