MinGW: Use -static-libgcc when linking a DLL.

 MinGW: Use -static-libgcc when linking a DLL.
 
 When commit 846e91e0223cf9f2821c3ad4dfffffbb929cb027 switched the linker
 driver from dlltool/dllwrap to gcc, it became possible for linking to
 choose shared libgcc.  Backends having loaded a module dynamically
 linked to libgcc can exit abnormally, which the postmaster treats like a
 crash.  Resume use of static libgcc exclusively, like 9.3 and earlier.
 Back-patch to 9.4.