[PATCH v2] rust: Respect HOSTCC when linking for host

[Matthew Maurer <mmaurer@google.com>]

Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is defined,
resulting in build failures in hermetic environments where `cc` does not
exist. This includes both hostprogs and proc-macros.

Since we are setting the linker to `HOSTCC`, we set the linker flavor to
`gcc` explicitly.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---

Updated the patch to reflect Nick's comment that KBUILD_HOSTLDFLAGS     
should be respected as well.

I did not switch it to use HOSTLD for two reasons:                     
* That variable is not globally defined - it is only available in two   
  subdirectories of tools/                                                                            
* C host scripts are linked by HOSTCC as well, even when linking a                                    
  collection of object files. It *prints* HOSTLD, but invokes HOSTCC.                                 
  See scripts/Makefile.host cmd_host-cmulti for an example.

 rust/Makefile         | 4 ++++
 scripts/Makefile.host | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 87958e864be0..b60b7eb8c5a0 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -380,9 +380,13 @@ $(obj)/exports_bindings_generated.h: $(obj)/bindings.o FORCE
 $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
 	$(call if_changed,exports)
 
+KBUILD_HOSTLDFLAGS_SQ = '$(subst ','\'',$(KBUILD_HOSTLDFLAGS))'
+
 quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
 	$(RUSTC_OR_CLIPPY) $(rust_common_flags) \
+		-Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		-Clink-args=$(KBUILD_HOSTLDFLAGS_SQ) \
 		--emit=dep-info=$(depfile) --emit=link=$@ --extern proc_macro \
 		--crate-type proc-macro \
 		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index 8f7f842b54f9..dc0410cae5ca 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -87,10 +87,14 @@ hostcxx_flags  = -Wp,-MMD,$(depfile) \
                  $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
                  $(HOSTCXXFLAGS_$(target-stem).o)
 
+KBUILD_HOSTLDFLAGS_SQ = '$(subst ','\'',$(KBUILD_HOSTLDFLAGS))'
+
 # `--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=$(HOSTCC) \
+		 -Clink-args=$(KBUILD_HOSTLDFLAGS_SQ) \
                  $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
                  $(HOSTRUSTFLAGS_$(target-stem))
 
-- 
2.42.0.459.ge4e396fd5e-goog