libre: linux-libre-64: fix strip
    
    Without that fix, on Parabola armv7h we have:
     |  Stripping vmlinux...
     |  strip:
     /build/linux-libre-64/pkg/linux-libre-64-headers/usr/lib/modules/5.11.11-gnu-2-64-ARCH/build/vmlinux:
     file format not recognized
     |  ==> ERROR: A failure occurred in package_linux-libre-64-headers().
     |      Aborting...
      ==> Copying log and package files out of the chroot...
    
    And after entering the chroot again, running file on vmlinux in
    usr/lib/modules/5.11.11-gnu-2-64-ARCH/build/ in
    build/linux-libre-64/pkg/linux-libre-64-headers/ gives the following:
      ELF 64-bit LSB shared object, ARM aarch64,
      version 1 (SYSV), statically linked,
      BuildID[sha1]=61b2acea5733ab812f33e872c462a54e7e71fbeb, not stripped
    
    So the file is not recognized because we use the host strip instead of
    the cross compiler's strip.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>