64-bit toolchain support

NDK r8e has a 64-bit toolchain available, so I updated the scripts to handle that nicely.

One change was to recognize 'x86_64' as well as 'amd64' when determining `$HOST_ARCH`. The net was to create the platform names as `[value]-$HOST_ARCH` instead of hardcoding `-x86`. The last change was to recognize a toolset name with a ` (64-bit)` suffix, although only for NDK r8e. I'm sure a more experienced shell-coding hand than mine can improve on my simple OR statement for that though…