Support TARGET_ARCH=armv6 and TARGET_ARCH=armv6eb

This adds a new TARGET_ARCH for building on ARM
processors that support the ARMv6K multiprocessor
extensions. In particular, these processors have
better support for TLS and mutex operations.

This mostly touches a lot of Makefiles to extend
existing patterns for inferring CPUARCH from ARCH.
It also configures:

• GCC to default to arm1176jz-s
• GCC to predefine FreeBSD_ARCH_armv6
• gas to default to ARM_ARCH_V6K
• uname -p to return 'armv6'
• make so that MACHINE_ARCH defaults to 'armv6'

It also changes a number of headers to use
the compiler ARM_ARCH_XXX macros to configure
processor-specific support routines.

I've successfully built and booted a BEAGLEBONE
image with these patches, though "make buildworld"
on the resulting image currently mis-compiles rtld.