[libc][POSIX] provide _POSIX_ARG_MAX in limits.h

[nickdesaulniers]

building llvm against llvm-libc fails with:

/llvm-project-main/llvm/lib/Support/Unix/Program.inc:561:24: error: use of undeclared identifier '_POSIX_ARG_MAX'

posix says this should go in limits.h and have a value of 4096. We should put this in:

libc/include/llvm-libc-macros/limits-macros.h

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

building llvm against llvm-libc fails with: ``` /llvm-project-main/llvm/lib/Support/Unix/Program.inc:561:24: error: use of undeclared identifier '_POSIX_ARG_MAX' ``` posix says this should go in limits.h and have a value of `4096`. We should put this in:

libc/include/llvm-libc-macros/limits-macros.h

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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3. Fix the issue locally.
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5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
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If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Nick Desaulniers (nickdesaulniers)

building llvm against llvm-libc fails with: ``` /llvm-project-main/llvm/lib/Support/Unix/Program.inc:561:24: error: use of undeclared identifier '_POSIX_ARG_MAX' ``` posix says this should go in limits.h and have a value of `4096`. We should put this in:

libc/include/llvm-libc-macros/limits-macros.h