kbuild: Add $(KBUILD_HOSTLDFLAGS) to 'has_libelf' test

Normally, invocations of $(HOSTCC) include $(KBUILD_HOSTLDFLAGS), which
in turn includes $(HOSTLDFLAGS), which allows users to pass in their own
flags when linking. However, the 'has_libelf' test does not, meaning
that if a user requests a specific linker via HOSTLDFLAGS=-fuse-ld=...,
it is not respected and the build might error.

For example, if a user building with clang wants to use all of the LLVM
tools without any GNU tools, they might remove all of the GNU tools from
their system or PATH then build with

$ make HOSTLDFLAGS=-fuse-ld=lld LLVM=1 LLVM_IAS=1

which says use all of the LLVM tools, the integrated assembler, and
ld.lld for linking host executables. Without this change, the build will
error because $(HOSTCC) uses its default linker, rather than the one
requested via -fuse-ld=..., which is GNU ld in clang's case in a default
configuration.

error: Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please
install libelf-dev, libelf-devel or elfutils-libelf-devel
make[1]: *** [Makefile:1260: prepare-objtool] Error 1

Add $(KBUILD_HOSTLDFLAGS) to the 'has_libelf' test so that the linker
choice is respected.

Link: https://github.com/ClangBuiltLinux/linux/issues/479


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>