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
diff --git a/core/combo/TARGET_linux-arm.mk b/core/combo/TARGET_linux-arm.mk
index fb390be..f0c35b6 100644
--- a/core/combo/TARGET_linux-arm.mk
+++ b/core/combo/TARGET_linux-arm.mk
@@ -197,7 +197,7 @@
 
 
 KERNEL_HEADERS_COMMON := $(libc_root)/kernel/uapi
-KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/uapi/asm-$(TARGET_ARCH)
+KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/uapi/asm-$(TARGET_$(combo_2nd_arch_prefix)ARCH)
 KERNEL_HEADERS := $(KERNEL_HEADERS_COMMON) $(KERNEL_HEADERS_ARCH)
 
 $(combo_2nd_arch_prefix)TARGET_C_INCLUDES := \