kernel-build: fix STRIP_KERNEL_EXPORTS for 64-bit kernels

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: 2ef0acc5fcda557fa5aaad35d27cb8cf75be96d2 "kernel-build: fix
STRIP_KERNEL_EXPORTS for recent kernels"

Signed-off-by: Rui Salvaterra <[email protected]>