tools/resolve_btfids: Support cross-building the kernel with clang

The CROSS_COMPILE variable may be present during resolve_btfids build if
the kernel is being cross-built. Since resolve_btfids is always executed
on the host, we set CC to HOSTCC in order to use the host toolchain when
cross-building with GCC. But instead of a toolchain prefix, cross-build
with clang uses a "-target" parameter, which Makefile.include deduces
from the CROSS_COMPILE variable. In order to avoid cross-building
libbpf, clear CROSS_COMPILE before building resolve_btfids.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20211216163842.829836-3-jean-philippe@linaro.org

tools/bpf/resolve_btfids/Makefile

@@ -19,6 +19,7 @@ CC       = $(HOSTCC)
 LD       = $(HOSTLD)
 ARCH     = $(HOSTARCH)
 RM      ?= rm
+CROSS_COMPILE =
 
 OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/