Remove BUILD_TARGET64

Remove BUILD_TARGET64

BUILD_TARGET64 creates a larger test matrix and mostly gates the
building of the aarch64/armv8 target, make that unconditional, which
would help fixing any issues with 64-bit address types anyway.

Rebased by Antonio Borneo after commit 1fbe8450a9dd ("mips: Add
MIPS64 support")

Change-Id: I219f62b744d540d9dde9a42e6b63fd7d91df3dbb
Suggested-by: Matthias Welwarsky <matthias@welwarsky.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5240
Tested-by: jenkins