tools/elfutils: pass -O2 in HOST_CXXFLAGS

tools/elfutils: pass -O2 in HOST_CXXFLAGS

Trying to compile elfutils on Fedora 40 with GCC 14.1.1 will fail with:
/home/robimarko/Building/AX3600/qualcommax/staging_dir/host/bin/g++ -std=c++11 -D_GNU_SOURCE -DHAVE_CONFIG_H -DLOCALEDIR='"/home/robimarko/Building/AX3600/qualcommax/staging_dir/host/share/locale"'  -DDEBUGPRED=0 -DSRCDIR=\"/home/robimarko/Building/AX3600/qualcommax/build_dir/host/elfutils-0.191/src\" -DOBJDIR=\"/home/robimarko/Building/AX3600/qualcommax/build_dir/host/elfutils-0.191/src\" -I. -I..  -I../libgnu -I../libgnu -I. -I. -I../lib -I.. -I./../libelf -I./../libebl -I./../libdw -I./../libdwelf -I./../libdwfl -I./../libasm -I../debuginfod -I/home/robimarko/Building/AX3600/qualcommax/staging_dir/host/include  -std=c++11 -Wall -Wshadow -Wtrampolines -Wlogical-op -Wduplicated-cond -Wnull-dereference -Wimplicit-fallthrough=5 -Werror -Wunused -Wextra -Wstack-usage=262144   -D_FORTIFY_SOURCE=3  -c -o srcfiles.o srcfiles.cxx
In file included from /usr/include/c++/14/x86_64-redhat-linux/bits/os_defines.h:39,
                 from /usr/include/c++/14/x86_64-redhat-linux/bits/c++config.h:2521,
                 from /usr/include/c++/14/cstdlib:41,
                 from ../libgnu/gettext.h:56,
                 from ../libgnu/eu-config.h:62,
                 from ../config.h:2378,
                 from srcfiles.cxx:31:
/usr/include/features.h:414:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
  414 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~
cc1plus: all warnings being treated as errors

So, lets do as the error says and pass -O2 in HOST_CXXFLAGS like we already
do by default in HOST_CFLAGS.

Link: https://github.com/openwrt/openwrt/pull/15368
Signed-off-by: Robert Marko <robimarko@gmail.com>