strace: fix build with BR2_LARGEFILE

strace: fix build with BR2_LARGEFILE

http://autobuild.buildroot.net/results/4ce0ce4dcb917889ef70208bd4bac63851c1fb92

Strace gets confused when built with -D_FILE_OFFSET_BITS=64 (which
transparently uses the lfs versions of functions/types), so strip this
from compiler/preprocessor flags.

Inspired by Gentoo who does the same:
http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/dev-util/strace/strace-4.7.ebuild

Tested using qemu_arm_versatile_defconfig and the following command line:
strace dd if=/dev/zero bs=1M skip=5000 count=1 of=/dev/null

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>