linker/xt-ld: Compute crtbegin/crtend paths in toolchain_linker_finalize

The computation of paths to crtbegin.o and crtend.o must be delayed until
after the compiler_file_path function has been defined. The
toolchain_linker_finalize function is where these are supposed to get set
now.

Signed-off-by: Keith Packard <keithp@keithp.com>