Don't pass irrelevant arguments to the compiler

Don't pass irrelevant arguments to the compiler

When building for 64-bit, don't pass options for 32-bit code generation,
and vice versa. This silences numerous warnings while building QtGui.

clang: warning: argument unused during compilation: '-Xarch_i386 -mmmx'
clang: warning: argument unused during compilation: '-Xarch_i386 -m3dnow'
clang: warning: argument unused during compilation: '-Xarch_i386 -mmmx'
clang: warning: argument unused during compilation: '-Xarch_i386 -m3dnow'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse'
clang: warning: argument unused during compilation: '-Xarch_i386 -mmmx'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse2'
clang: warning: argument unused during compilation: '-Xarch_i386 -m3dnow'
clang: warning: argument unused during compilation: '-Xarch_i386 -mmmx'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse2'
clang: warning: argument unused during compilation: '-Xarch_i386 -m3dnow'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse'
clang: warning: argument unused during compilation: '-Xarch_i386 -mssse3'
clang: warning: argument unused during compilation: '-Xarch_i386 -mssse3'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse2'
clang: warning: argument unused during compilation: '-Xarch_i386 -msse2'
clang: warning: argument unused during compilation: '-Xarch_i386 -mssse3'
clang: warning: argument unused during compilation: '-Xarch_i386 -mssse3'

Change-Id: If4313b151e73db88d516998e3cf546e62495c289
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>