Modify stdio to support Trusty kernel

This commit modifies musl's stdio to support the kernel by

1. replacing the syscall in __stdio_write with a call to writev
2. removing support for the floating-point code in stdio
3. replacing __pthread_self with current_thread_libc_state
4. redefining macros that don't apply to stdout/stderr (e.g. FLOCK)
   to avoid needing to compile/stub out pthread sources

Bug: 230134581
Change-Id: I985496b6fc62176ff33ad5ebb90c694cd5309977
10 files changed