mesa: add managarm support

This MR adds support for the managarm operating system. Managarm is a microkernel-based operating system, however, as it (or, to be more accurate, its libc, mlibc) implements some Linux-specific APIs, with the goal of being source-compatible with Linux software, porting mesa does not require many changes.

In general, we basically only add ourselves to existing code paths, however we do have to disable a code path, specifically the UTIL_FUTEX_SUPPORTED code path, since it assumes that Linux system calls are available, which is obviously not the case, while only checking for the existence of a Linux header, which is present.