kernel-build: fix STRIP_KERNEL_EXPORTS for 64-bit kernels

While parsing the nm output, we need to account for the fact that 64-bit kernels
have 64-bit wide addresses. While at it, replace the grep | sed combo with a
single awk invocation and a stronger regex.

Fixes: 2ef0acc "kernel-build: fix
STRIP_KERNEL_EXPORTS for recent kernels"

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

include/kernel-build.mk

@@ -105,7 +105,7 @@ define BuildKernel
 		xargs $(TARGET_CROSS)nm | \
 		awk '$$$$1 == "U" { print $$$$2 } ' | \
 		sort -u > $(KERNEL_BUILD_DIR)/mod_symtab.txt
-	$(TARGET_CROSS)nm -n $(LINUX_DIR)/vmlinux.o | grep ' [rR] __ksymtab' | sed -e 's,........ [rR] __ksymtab_,,' > $(KERNEL_BUILD_DIR)/kernel_symtab.txt
+	$(TARGET_CROSS)nm -n $(LINUX_DIR)/vmlinux.o | awk '/^[0-9a-f]+ [rR] __ksymtab_/ {print substr($$$$3,11)}' > $(KERNEL_BUILD_DIR)/kernel_symtab.txt
 	grep -Ff $(KERNEL_BUILD_DIR)/mod_symtab.txt $(KERNEL_BUILD_DIR)/kernel_symtab.txt > $(KERNEL_BUILD_DIR)/sym_include.txt
 	grep -Fvf $(KERNEL_BUILD_DIR)/mod_symtab.txt $(KERNEL_BUILD_DIR)/kernel_symtab.txt > $(KERNEL_BUILD_DIR)/sym_exclude.txt
 	( \