freebsd.libcxxrt: don't copy unwind headers

These headers conflict with the unwind headers from libunwind
provided by libgcc_eh in `freebsd.libc`.
Upstream FreeBSD does not use these headers in any capacity,
and they cause some incompatibilities since libcxxrt unwind.h
requires _GNU_SOURCE for some functions, while libunwind does not.

pkgs/os-specific/bsd/freebsd/patches/14.0/libcxxrt-headers.patch

@@ -4,7 +4,7 @@
  SRCS+=		terminate.cc
  SRCS+=		typeinfo.cc
  
-+INCS+=cxxabi.h unwind.h unwind-arm.h unwind-itanium.h
++INCS+=cxxabi.h
 +
  WARNS?=		0
  CFLAGS+=	-isystem ${SRCDIR} -nostdinc++