I merged an old change of @minux's (4d7a5ed) which printed external linker output so warnings aren't missed.

That broke arm64:
https://build.golang.org/log/6be7927bc50f92753c05d648ce488271487b4f09

... the build is full of warnings like:

/usr/bin/ld: /tmp/go-link-697234120/go.o: warning: linking abicalls files with non-abicalls files

But a test at the end is comparing output against expected golden output and failing after seeing "warning: linking abicalls files with non-abicalls files" in the test output.

/cc @ianlancetaylor @MIPSbkirby @minux