bpf: Fix cross build for CONFIG_DEBUG_INFO_BTF option

Stephen and 0-DAY CI Kernel Test Service reported broken cross build
for arm (arm-linux-gnueabi-gcc (GCC) 9.3.0), with following output:

/tmp/ccMS5uth.s: Assembler messages:
/tmp/ccMS5uth.s:69: Error: unrecognized symbol type ""
/tmp/ccMS5uth.s:82: Error: unrecognized symbol type ""

Having '@object' for .type diretive is wrong because '@' is comment
character for some architectures. Using STT_OBJECT instead that should
work everywhere.

Also using HOST* variables to build resolve_btfids so it's properly
build in crossbuilds (stolen from objtool's Makefile).

Reported-by: kernel test robot <[e-mail]>
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Jiri Olsa <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Tested-by: Geert Uytterhoeven <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/btf_ids.h

···
 asm(							\
 ".pushsection " BTF_IDS_SECTION ",\"a\";       \n"	\
 ".local " #symbol " ;                          \n"	\
-".type  " #symbol ", @object;                  \n"	\
+".type  " #symbol ", STT_OBJECT;               \n"	\
 ".size  " #symbol ", 4;                        \n"	\
 #symbol ":                                     \n"	\
 ".zero 4                                       \n"	\

tools/bpf/resolve_btfids/Makefile

···
   MAKEFLAGS=--no-print-directory
 endif
 
+# always use the host compiler
+ifneq ($(LLVM),)
+HOSTAR  ?= llvm-ar
+HOSTCC  ?= clang
+HOSTLD  ?= ld.lld
+else
+HOSTAR  ?= ar
+HOSTCC  ?= gcc
+HOSTLD  ?= ld
+endif
+AR       = $(HOSTAR)
+CC       = $(HOSTCC)
+LD       = $(HOSTLD)
+
 OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
 
 LIBBPF_SRC := $(srctree)/tools/lib/bpf/