build: add more precious variables for cross-compilation

build: add more precious variables for cross-compilation

Using AC_ARG_VAR function, it's possible to add LD, AR, OBJCOPY,
OBJDUMP, RANLIB and STRIP to the list of precious variables in
$ac_precious_vars for configure tool.

Doing this, we are enabling a new set of variables (HOST_LD, HOST_AR,
HOST_OBJCOPY, HOST_OBJDUMP, HOST_RANLIB and HOST_STRIP) useful for
cross-compiling when the target toolchain is prepending a prefix
to these tools.

Signed-off-by: Emanuele Bovisio <emanuele.bovisio@eolo.it>