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.

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

The previous implementation of the ctype functions assumed ASCII.
This patch changes to a switch/case implementation that looks odd, but
actually is easier for the compiler to understand and optimize.

Implements std::from_chars for float and double.

The implementation uses LLVM-libc to do the real parsing. Since this is
the first time libc++
uses LLVM-libc there is a bit of additional infrastructure code. The
patch is based on the
[RFC] Project Hand In Hand (LLVM-libc/libc++ code sharing)

https://discourse.llvm.org/t/rfc-project-hand-in-hand-llvm-libc-libc-code-sharing/77701

Needed for #110894

The strtointeger code was updated to support some bigint types in #85201
but not all. This patch finishes the cleanup so that it can work for a
wider range of types.

This is a part of #97655.

Reverts llvm/llvm-project#98075

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