[GR-74988] Base-relative code pointers and PIE by default.

[graalvmbot]

• Default to building position-independent executables (PIE). They have security advantages and are enabled by default in the toolchains of many Linux OSes, but not all. Even then, their software packages typically use PIE. macOS and Windows toolchains have turned on PIE for some time already.
• Enable RelativeCodePointers by default. For PIE and shared libraries, this substantially reduces the number of relocations and therefore image size and memory usage (and improves sharing between processes). This comes at the cost of a dedicated code base register and extra address arithmetic, which turns out to be near-negligible.