[compiler-rt] [compiler-rt][common] Only unmap stacks the runtime has actually mapped (PR #179000)

[via llvm-commits]

accauble wrote:

Hi there. I found the same bug and made a [similar fix before finding your PR](https://github.com/llvm/llvm-project/commit/2feb5203f9924e9bdd7fd7150f00c3d71c9d9411). I have another test case. My changes were slightly different:

* I chose to disable the signal handling only if the registered stack is the one we allocated (so we don't clobber whoever registered it)
* I chose to unmap our allocated stack no matter what
* I chose to use a static variable to remember the stack pointer which kept the changes to one file.

https://github.com/llvm/llvm-project/pull/179000