toolchain: mold: add PKG_NAME to Makefile

toolchain: mold: add PKG_NAME to Makefile

In include/host-build.mk, HOST_BUILD_DIR is set by default value:
HOST_BUILD_DIR ?= $(BUILD_DIR_HOST)/$(PKG_NAME)

However the mold package has no PKG_NAME set at all. This means the
HOST_BUILD_DIR is identical to $(BUILD_DIR_HOST).

In the Host/Prepare stage, by default, the $(HOST_BUILD_DIR) will be
deleted at first unconditionally. Since HOST_BUILD_DIR is identical
to $(BUILD_DIR_HOST), the entire build_dir/toolchain-* directory will
be removed and this will cause build failure.

Adding PKG_NAME:=ld.mold can solve this issue.

Signed-off-by: Weijie Gao <hackpascal@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18567
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit b9cb07eced1de81b647608ee89a95d925c7c12a8)