build: Boostrap LLVM_BINUTILS for cross-tools

[emaste]

Update WITHOUT_LLVM_BINUTILS_BOOTSTRAP description.

[emaste]

It looks like the cross build is currently broken for other reasons

/Users/runner/work/freebsd-src/freebsd-src/contrib/tzcode/zic.c:1574:10: error: call to undeclared library function 'mempcpy' with type 'void *(void *, const void *, unsigned long)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 1574 |     cp = mempcpy(cp, src, dirlen);
      |          ^
/Users/runner/work/freebsd-src/freebsd-src/contrib/tzcode/zic.c:1574:10: note: include the header <string.h> or explicitly provide a declaration for 'mempcpy'

[emaste]

Two failures:

• Cross-build Kernel / aarch64 macos-latest (clang-18) Failed in 19 minutes and 24 seconds
• Cross-build Kernel / amd64 macos-latest (clang-18) Failed in 5 minutes and 15 seconds

All four Ubuntu cross-build jobs passed

[emaste]

Pull request for testing at https://github.com/freebsd/freebsd-src/pull/2084