to be installed when linking libc.

When zig links against libc, it requires a dynamic linker path.
Usually this can be determined based on the architecture and operating
system components of the target. However on some systems this is not
correct; because of this zig checks its own dynamic linker.

When zig is statically linked, this information is not available, and so
it resorts to using cc -print-filename=foo to find the dynamic linker
path.

Before this commit, Zig incorrectly exited with an error if there was
no c compiler installed. Now, Zig falls back to the dynamic linker
determined based on the arch and os when no C compiler can be found.