Bazel only supports overlay build currently, so there's always system
libc (and system errno_ present. Use the new
LIBC_ERRNO_MODE_SYSTEM_INLINE by default, instead of using thread-local
one in the unit tests and system errno for "public packaging" (i.e. prod
build). This way, we can ensure that we're testing the same
configuration as the one that will be used in production.
This is the second attempt to do that after f9146cc was reverted in
279e82f. Since then, many tests have been migrated to ErrnoCheckingTest
so the change should be safe even for those downstream customers that
clobber system errno value before unit tests are being called.
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