powerpc/tools: Pass -mabi=elfv2 to gcc-check-mprofile-kernel.sh

Toolchains don't always default to the ELFv2 ABI. This is true with at
least the kernel.org toolchains. As such, pass -mabi=elfv2 explicitly to
ensure that we are testing against the correct compiler output.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
[mpe: Tweak comment wording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230530093821.298590-1-naveen@kernel.org