[Bug other/121307] Parallel build issue with bootstrap-ubsan

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121307

--- Comment #1 from Sam James <sjames at gcc dot gnu.org> ---
In bootstrap-ubsan.mk, we do:
```
POSTSTAGE1_LDFLAGS += -fsanitize=undefined -static-libubsan -DUBSAN_BOOTSTRAP \
                      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ \
                      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ubsan/ \
                      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ubsan/.libs
```

I think those -B workaround the problem for everything but stage3? You can't
hit this in stage2 because of those lines, at least.