makefile: Remove unused FBC_ASM/FBC_C variables
makefile: Simplify some cases of rm -f/rmdir to just rm -rf
makefile: Fix uninstall-rtlib to correctly remove libc.a under DOS
makefile: Remove unnecessary bindir variable

For both normal and standalone builds we need a bin/ directory, so we can
just always create it, and don't need a variable for it.
makefile: Build -fPIC versions of FB libs for non-x86 Unixes
Add -pic command line option
Link with PIC versions of FB libs under -pic
Use crtbeginS.o and crtendS.o under -pic, instead of the normal versions
Pass -pic on to gcc/llc
Add changelog entries about PIC stuff