toolchain/helpers.mk: update check_arm_abi comment

Since commit b2e88073 (toolchain: check ARM EABI vs. EABIhf for external
toolchains), check_arm_abi accepts a second parameter. Update the comment
accordingly.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>