The TF-A firmware build system is capable of building on both Unix like
and Windows host PCs. The commit ID 7ff088 "Enable MTE support" updated
the Makefile to conditionally enable the MTE support if the AArch64
architecture revision was greater than 8.5. However, the Makefile changes
were dependent on shell commands that are only available on unix shells,
resulting in build failures on a Windows host PC.

This patch fixes the Makefile by using a more portable approach for
comparing the architecture revision.

Change-Id: Icb56cbecd8af5b0b9056d105970ff4a6edd1755a
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>