toolchain: check ARM EABI vs. EABIhf for external toolchains

Following the introduction of the support of EABIhf as a second ARM
ABI, it is important to check whether the external toolchain provided
by the user actually uses the ABI that has been selected in the
Buildroot configuration. This commit introduces such a check by
looking at the 'Tag_ABI_VFP_args' tag of the architecture-specific
section of the ELF headers. This assumes that ELF is the binary format
used on ARM, which may not be the case on ARM noMMU systems (they use
the FLAT binary format), but Buildroot doesn't have support for such
systems at the moment.

Also ensure the correct CFLAGS are passed to the cross compiler for the
test, so the correct variant is used in case the toolchain is multilib.

[Peter: mention CFLAGS change]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>