* Some GCC flags improvements
* Move -fno-reorder-blocks -fno-zero-initialized-in-bss to assets only
* Add -fno-PIC since some gcc versions use -fPIC as default
* Enable builtin functions on gcc, with appropriate changes to missing_gcc_functions.c
and ultra64/libc.h
* Move -fno-merge-constants -mno-explicit-relocs -mno-split-addresses to overlays only
as they are only needed there for reloc reasons
* Remove unneeded casts in missing_gcc_functions.c
* Change gcc assets flags handling
3670a48aee