Support build when ccache is used

[ycongal-smile]

Currently, a ccache enabled build (i.e. HOSTCC="ccache $compiler") fails with:

| HOSTRUSTC scripts/generate_rust_target
| HOSTCC scripts/kallsyms
| HOSTCC scripts/sorttable
| HOSTCC scripts/asn1_compiler
| TOUCH include/generated/gcc-plugins.h
| DESCEND objtool
| error: multiple input filenames provided (first two filenames are gcc and
.../scripts/generate_rust_target.rs)

This is caused by this line: https://github.com/Rust-for-Linux/linux/blob/b8d687c7eeb52d0353ac27c4f71594a2e6aa365f/scripts/Makefile.host :

# `--out-dir` is required to avoid temporaries being created by `rustc` in the
# current working directory, which may be not accessible in the out-of-tree
# modules case.
hostrust_flags = --out-dir $(dir $@) --emit=dep-info=$(depfile) \
		 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
                 $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
                 $(HOSTRUSTFLAGS_$(target-stem))

Surely, -Clinker=$(HOSTCC) should be escaped to to -Clinker="$(HOSTCC)" but that does not fix the problem because rustc interpret the -Clinker option as a path and not a command.

Notes:

• we are trying to fix this rustc limitation in rustc_codegen_ssa: fix pre_link_args order rust-lang/rust#151165
• This was triggered when trying to add rust support to the Yocto kernel. More details can be found on the Yocto ML.

Cc: @alban-moizan-smile

[mqqz]

I submitted a patch which should hopefully address this.