PlatformIO: Fix build on posix systems, fix spaces in project path

[multiplemonomials]

Summary of changes

Found three small bugs in the PlatformIO build script:

• The path to libmbed-os.a had hardcoded backslashes, breaking builds on non-Windows platforms (oops!)
• The code that processed build flags from CMake was erroneously stripping away quotes, causing -include "Some Path" to be turned into -include Some Path, which then got processed into -include Some, and then Path. We can fix this by just leaving the quotes in place, allowing them to be passed through to Click.
• Similarly, the code that generated the libmbed-os.a path was not quoting the path, causing a build failure if the path contained spaces (I didn't realize that SCons wasn't smart enough to do this automatically!)

Impact of changes

A few build errors with platformio will be fixed!

Migration actions required

Documentation

New doc page for platformio created here: https://mbed-ce.dev/getting-started/platformio-setup/

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Able to compile the project now when the build dir contains spaces