build: use arm kernel headers for arm 2nd arch builds

Compiling for arm requires using the arm kernel headers and not
the aarch64 kernel headers.  Add $(combo_2nd_arch_prefix) to get
asm-$(TARGET_2ND_ARCH) when arm is the 2nd arch.

Change-Id: I15270d0ef35e48c034bf4d0d5e35b76f67b2a8e1