Do not add host architecture if an arm architecture is set

[tobil4sk]

Currently if HXCPP_ARMV7 is set for example, hxcpp ignores it and adds the host architecture as if no architecture were set. This causes problems, for example, because it might result in both HXCPP_ARMV7 and HXCPP_ARM64 being set at once, which breaks some toolchains:

hxcpp/toolchain/iphoneos-toolchain.xml

Lines 43 to 47 in 18f87b9

	<flag value="-arch"/>
	<flag value="armv6" if="HXCPP_ARMV6" />
	<flag value="armv7" if="HXCPP_ARMV7" />
	<flag value="armv7s" if="HXCPP_ARMV7S" />
	<flag value="arm64" if="HXCPP_ARM64" />

(This leads to -arch armv7 arm64 being passed to the compiler, which causes Error: clang: error: no such file or directory: ‘arm64’).

This patch prevents the host architecture being added if any of the following are defined: HXCPP_ARMV6, HXCPP_ARMV7, HXCPP_ARMV7S.

[tobil4sk]

@Simn green here now also, thanks.