packaging: Enable cross-building agent

Requires setting ARCH and CC.

- Add CC linker option for building agent.
- Set host for building libseccomp.

Fixes: #3681
Signed-off-by: Jakob Naucke <jakob.naucke@ibm.com>

utils.mk

@@ -113,7 +113,8 @@ endef
 BUILD_TYPE = release
 
 ##VAR ARCH=arch target to build (format: uname -m)
-ARCH = $(shell uname -m)
+HOST_ARCH = $(shell uname -m)
+ARCH ?= $(HOST_ARCH)
 ##VAR LIBC=musl|gnu
 LIBC ?= musl
 ifneq ($(LIBC),musl)
@@ -142,6 +143,14 @@ ifeq ($(ARCH), aarch64)
     $(warning "WARNING: aarch64-musl needs extra symbols from libgcc")
 endif
 
+ifneq ($(HOST_ARCH),$(ARCH))
+    ifeq ($(CC),)
+         CC = gcc
+         $(warning "WARNING: A foreign ARCH was passed, but no CC alternative. Using gcc.")
+    endif
+    override EXTRA_RUSTFLAGS += -C linker=$(CC)
+endif
+
 TRIPLE = $(ARCH)-unknown-linux-$(LIBC)
 
 CWD := $(shell dirname $(realpath $(firstword $(MAKEFILE_LIST))))