package/rust: use host libraries from HOST_DIR

host-rust package depends on several host packages to provide tools and
libraries but it doesn't take into account out host libraries in
HOST_DIR while building rustc compiler. Indeed, rustc needs zlib and
fails to link if zlib is not installed on the host.

  error: could not compile `rustc_driver` (lib) due to previous error

If zlib is installed on the host, we can notice it with ldd tool (while
it should be linked with the one provided by Buildroot host-zlib):

  ldd [...]TestRust/host/bin/rustc
  libz.so.1 => /lib64/libz.so.1

Provide HOST_LDFLAGS using llvm.ldflags in config.toml.
(HOST_LDFLAGS provides -L$(HOST_DIR)/lib -Wl,-rpath,$(HOST_DIR)/lib)

With that fixed, rustc_driver link with libz from HOST_DIR but the
host-rust build still fail later due to another issue.

  error: could not compile `rustdoc-tool` (bin "rustdoc_tool_binary") due to previous error

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/6256881545
http://autobuild.buildroot.org/results/a6b/a6b28783f29e6b729824bf42679a62f72ad5bee0

Signed-off-by: Romain Naour <romain.naour@smile.fr>
[yann.morin.1998@free.fr: slight rewording in commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/rust/rust.mk

@@ -65,6 +65,7 @@ define HOST_RUST_CONFIGURE_CMDS
 		echo 'cc = "$(TARGET_CROSS)gcc"'; \
 		echo '[llvm]'; \
 		echo 'ninja = false'; \
+		echo 'ldflags = "$(HOST_LDFLAGS)"'; \
 	) > $(@D)/config.toml
 endef