Fix windows compilation issue in cmake/libutils/save_linker_opts.cc

[lpinto-es]

Fixed a compilation issue in Windows where "unistd.h" was still being used to call "getcwd".
Protected the "unistd.h" with a #ifdef WIN32 macro and added a windows alternative using the #include <direct.h> and the _getcwd counterpart for that OS.

I was getting this issue while compiling with Ninja, and after those changes everything now compiles as it should (against openssl 1.1.1q using VS 2019 (latest update) on a Windows 11 intel machine!)

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[lpinto-es]

Already signed the Oracle Contribution Agreement (OCA)

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[lpinto-es]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

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