document (wikify) common porting issues and quick solutions

[treeswift]

• MAKE_SHARED_FROM_STATIC
• MXE_INTRINSIC_SH
• ARM/Thumb in GMP
• disabling tests in automake projects when cross-compiling
• TBA

[treeswift]

Ongoing; keeping open until (a) ARM/Thumb porting is documented; (b) GUI components are reviewed and verified.