[musl] Clean out more linux syscall goop

- Remove some more unused SYS_* #defines
- Remove some dead internal wrappers around syscalls
- Remove Linux's prctl
- Move __syscall(SYS_fcntl, ...) to fcntl(...)
- Move __syscall(SYS_exit, ...) to exit(...)

Change-Id: I017599ce5afb6bf78021be0c71824056e16c1f65

[musl] Remove linux-y LFS64 abstraction and xstat extension

Change-Id: Id54c438c2b36d7fce3adb9bd7d32045bbc0765fb

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit