Fix two instances of libcxx_shared_headers depending on .cpp files (in
Bazel build):

* Don't depend on exit syscall in LIBC_ASSERT implementation. This
dependency is not used, since LIBC_ASSERT always uses system <assert.h>
in the overlay mode, which is the only mode supported by Bazel.
* Don't depend on libc_errno in str-to-float and str-to-integer
conversions. We only need the ERANGE value, which can be obtained from
the proxy header instead.

Summary:
This uses `internal::exit` which is not built in overlay mode, leading
to linker errors. Fix this to just use `assert.h`.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

- In /libc/src/__support/ OSUtil, changed quick_exit to just exit, and
put in namespace
LIBC_NAMESPACE::internal.
- In /libc/src/stdlib added quick_exit
- Added test files for quick_exit

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This patch is an alternative to D155902. It provides the following benefits:
 - No buffer manual allocation and error handling for the general case
 - More flexible API : width specifier, sign and prefix handling
 - Simpler code

The more flexible API removes the need for manually tweaking the buffer afterwards, and so prevents relying on implementation details of IntegerToString.

Reviewed By: michaelrj, jhuber6

Differential Revision: https://reviews.llvm.org/D156981