Correctly determine libc.so 'OUTPUT_FORMAT' when cross-compiling.

Commit 87d583c6 replaces the sed script
with an "objdump -f" invocation to determine the 'OUTPUT_FORMAT' bit of
the libc.so linker script.

However, when cross-compiling, for example from x86_64-linux-gnu to
aarch64-linux-gnu, "objdump -f" would report the wrong
format ("elf64-little").  Conversely, "aarch64-linux-gnu-objdump -f"
reports "elf64-littleaarch64" as expected.

This patch changes 'configure.ac' to use AC_CHECK_TOOL rather than
'$CC -print-prog-name=objdump' to determine the value of the OBJDUMP
variable.  That way, OBJDUMP is set to TRIPLET-objdump when
cross-compiling for TRIPLET.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>