bsd-user: add arm target build

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Acked-by: Kyle Evans <kevans@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

configs/targets/arm-bsd-user.mak

+TARGET_ARCH=arm
+TARGET_XML_FILES= gdb-xml/arm-core.xml gdb-xml/arm-vfp.xml gdb-xml/arm-vfp3.xml gdb-xml/arm-vfp-sysregs.xml gdb-xml/arm-neon.xml gdb-xml/arm-m-profile.xml gdb-xml/arm-m-profile-mve.xml