TFTF: Add build option for Arm Feature Modifiers

This patch adds a new ARM_ARCH_FEATURE build option
to add support for compiler's feature modifiers.
It has the form '[no]feature+...' and defaults to
'none'. This option translates into compiler option
'-march=armvX[.Y]-a+[no]feature+...'.

Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Change-Id: I115a921c777b7932523d2dff8e8e03377d87bb78
3 files changed