Adds l64a, which generates the base 64 string expected by a64l.

Implements the posix-specified strftime conversions for the default
locale, along with comprehensive unit tests. This reuses a lot of design
from printf, as well as the printf writer.

Roughly based on #111305, but with major rewrites.

This PR:
- add entrypoints for `pthread_spin_*`
- additionally, the fixes a typo that has been disabling lock related
tests

Reverts llvm/llvm-project#98989

migrated from https://github.com/llvm/llvm-project/pull/95965 due to
corrupted git history

Adds `dlopen` and friends. This is needed as part of the effort to
compile `libunwind` + `libc` without baremetal mode. This is part of
https://github.com/llvm/llvm-project/issues/97191. This should still be
spec compliant, since `dlopen` always returns `NULL` and `dlerror`
always returns an error message.

> If dlopen() fails for any reason, it returns NULL.

> The function dlclose() returns 0 on success, and nonzero on error.

> Since the value of the symbol could actually be NULL (so that a NULL
return from dlsym() need not indicate an error), the correct way to test
for an error is to call dlerror() to clear any old error conditions,
then call dlsym(), and then call dlerror() again, saving its return
value into a variable, and check whether this saved value is not NULL.


See:
- https://linux.die.net/man/3/dlopen