objtool: Allow AR to be overridden with HOSTAR

Currently, this Makefile hardcodes GNU ar, meaning that if it is not
available, there is no way to supply a different one and the build will
fail.

  $ make AR=llvm-ar CC=clang LD=ld.lld HOSTAR=llvm-ar HOSTCC=clang \
         HOSTLD=ld.lld HOSTLDFLAGS=-fuse-ld=lld defconfig modules_prepare
  ...
    AR       /out/tools/objtool/libsubcmd.a
  /bin/sh: 1: ar: not found
  ...

Follow the logic of HOST{CC,LD} and allow the user to specify a
different ar tool via HOSTAR (which is used elsewhere in other
tools/ Makefiles).

Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Reviewed-by: Mukesh Ojha <[email protected]>
Cc: <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/80822a9353926c38fd7a152991c6292491a9d0e8.1558028966.git.jpoimboe@redhat.com
Link: ClangBuiltLinux#481
Signed-off-by: Ingo Molnar <[email protected]>

Author: nathanchance

tools/objtool/Makefile

@@ -7,11 +7,12 @@ ARCH := x86
 endif
 
 # always use the host compiler
+HOSTAR	?= ar
 HOSTCC	?= gcc
 HOSTLD	?= ld
+AR	 = $(HOSTAR)
 CC	 = $(HOSTCC)
 LD	 = $(HOSTLD)
-AR	 = ar
 
 ifeq ($(srctree),)
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))