only use --gc-sections for release builds, because it can harm debugging

[DanAlbert]

Description

#1717 needs a partial revert

[DanAlbert]

The rationale from Studio:

It can cause unused functions to be removed even with -O0, which causes surprises when trying to use those functions in expression evaluation.

[DanAlbert]

This can't be fixed for the new CMake toolchain, only the legacy one (which is fortunately the default), because CMake doesn't have an affordance for us to override ldflags for specific build modes: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/Platform/Android-Clang.cmake#L16-21

r25c will fix ndk-build and the legacy cmake toolchain file (again, the default), but the new form is going to remain broken until someone can be freed up to spend the time needed to make cmake work for us.

[DanAlbert]

Forked the new toolchain file stuff into #1826.