FROMLIST: kbuild: improve libelf detection

When `pkg-config --libs' wasn't used or didn't return anything useful,
the test for libelf would fail even if the headers and libraries were
available to kbuild via KBUILD_CFLAGS and KBUILD_LDFLAGS.

This makes the check in the core Makefile match the check that is done
in tools/objtool/Makefile. Specifically, the C compiler is passed the
full KBUILD_CFLAGS and KBUILD_LDFLAGS, which may have set up additional
sysroot/include/library search paths to find libelf.

Signed-off-by: Alistair Delva <adelva@google.com>
Link: https://lore.kernel.org/lkml/20210112190010.1582888-1-adelva@google.com/
Bug: 175705438
Change-Id: I06b0997c2df8545877d8b2d29b7a3045551c92e2