toolchain-external: Fix EABIhf check

Currently the check for EABI/EABIhf toolchains looks for the
Tag_ABI_VFP_args attribute in the crt1.o file which gcc adds in an
EABIhf toolchain.
In uClibc, however, crt1.o is not compiled from c but assembly, so the
Tag_ABI_VFP_args attribute is not added in the object file. This causes
the EABIhf check in the external toolchain logic to fail for
uClibc-based toolchains.

Fix by compiling a dummy .c file and trying to link the object against the
C library. Since it is impossible to mix EABI and EABIhf code, a mismatch
between the buildroot and toolchain ABI settings will be detected during
this link step.

Fixes bug #6842: https://bugs.busybox.net/show_bug.cgi?id=6842



[Peter: fix final 'fi']
Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
[ThomasDS: do full link iso readelf test, update commit message]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>