Fix lib/libc for object tree on case-insensitive file system.

[marcel]

Summary

When MAKEOBJDIRPREFIX points to a case-insensitive file system, the build can break when different source files create the same object files (case-insensitivity speaking).

[bdrewery]

Is this the only issue in the tree?

[sjg]

libc is one of the places that creates multiple files that only vary by one letter (case).
I'd be surprised if this is the only place that breaks, but fixing them one at a time is better than nothing.

[marcel]

Adding my response to bdrewery@ for posterity:

No, just the first one (in a sequential build). The next one is in libcasper. I want to address them 1 at a time, because we may want different fixes for different libraries.