[Driver] Don't override '-march' when using '-arch x86_64h'

On Darwin, using '-arch x86_64h' would always override the option passed
through '-march'.

This patch allows users to use '-march' with x86_64h, while keeping the
default to 'core-avx2'

Differential Revision: https://reviews.llvm.org/D55775

llvm-svn: 349381