memory_footprint.py: correct the toolchain name

The toolchain name has been changed from toolchain_GNUARM.cmake to
GCC_7_3_1 on the commit 433771e96f4

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I93b054cd7b0d027282a3e973e47ab0d4fe4f3814
1 file changed