When used in an ifunc resolver, errno@plt won't be available. This is
the API the rivos folks contributing to glibc are leaning towards, for
the same reason. Hit by the berberis folks because they don't implement
the syscall so they were trying to set errno to ENOSYS.
Tested by looking at the generated assembler, and also disabling the
vdso (since on actual systems, this will go via the vdso).
Test: treehugger
Change-Id: Ie2779110f141f20efe97cb892fbdefd808b5339b
Also fix a __has_include() that was copy & pasted without modification
from a different test.
Test: treehugger
Change-Id: Iafceb331a95ec788596ffdad60aa2202b692e589
