bsd.port.mk: Make WITHOUT_CPU_CFLAGS also work for CXXFLAGS

In base 0fb33e049ac0 the _CPUFLAGS value (initialized from CPUTYPE in
make.conf) is also being appended to CXXFLAGS, instead of only to
CFLAGS.

Some ports use WITHOUT_CPU_CFLAGS to avoid having CPUTYPE flags added to
their command lines, for example devel/wasi-libcxx*.

bsd.port.mk handles it like so:

. if defined(WITHOUT_CPU_CFLAGS)
. if defined(_CPUCFLAGS)
. if !empty(_CPUCFLAGS)
CFLAGS:= ${CFLAGS:C/${_CPUCFLAGS}//}
. endif
. endif
. endif

However this will no longer work after base 0fb33e049ac0 (which is in
main and stable/15), leading to build errors similar to:

FAILED: [code=1] libcxxabi/src/CMakeFiles/cxxabi_static_objects.dir/cxa_aux_runtime.cpp.o
/usr/local/llvm19/bin/clang++ --target=wasm32-wasi --sysroot=/usr/local/share/wasi-sysroot -DLIBCXXABI_SILENT_TERMINATE -DLIBCXX_BUILDING_LIBCXXABI -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCXXABI_BUILDING_LIBRARY -D_LIBCXXABI_HAS_NO_THREADS -DSTDC_CONSTANT_MACROS -DSTDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/wrkdirs/usr/ports/devel/wasi-libcxx19/work/llvm-project-19.1.7.src/libcxxabi/../libcxx/src -I/wrkdirs/usr/ports/devel/wasi-libcxx19/work/.build/include/c++/v1 -I/wrkdirs/usr/ports/devel/wasi-libcxx19/work/llvm-project-19.1.7.src/libcxxabi/include -O2 -pipe -fno-strict-aliasing -march=haswell -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O2 -pipe -fno-strict-aliasing -march=haswell -DNDEBUG -std=c++23 -D_DEBUG -UNDEBUG -MD -MT libcxxabi/src/CMakeFiles/cxxabi_static_objects.dir/cxa_aux_runtime.cpp.o -MF libcxxabi/src/CMakeFiles/cxxabi_static_objects.dir/cxa_aux_runtime.cpp.o.d -o libcxxabi/src/CMakeFiles/cxxabi_static_objects.dir/cxa_aux_runtime.cpp.o -c /wrkdirs/usr/ports/devel/wasi-libcxx19/work/llvm-project-19.1.7.src/libcxxabi/src/cxa_aux_runtime.cpp
clang++: error: unsupported option '-march=' for target 'wasm32-wasi'

Fix this by using the same logic for CXXFLAGS as for CFLAGS.

PR: 289844, 289673
Approved by: portmgr timeout (2 weeks)
MFH: 2025Q4

(cherry picked from commit 9d078a8bc60d0528b74b88f19b61911818d3a415)