Fixes#527.
Previously on Linux, '__$startup_runtime', the procedure that
initializes the type table for runtime typeids, was NOT called when
a shared library was loaded by the dynamic loader.
This caused the library to not have its type table populated, which
caused an assert to fail if you tried to use runtime typeids - like
core:fmt, for example.
This commit fixes this by calling ld instead of clang, when building a
shared library, so that we can pass "-init '__$startup_runtime'" to it,
when building a shared library.
Try as I might, I could not get clang to correctly pass through
the linker flags that I wanted.
140bb3ebfc