tccgen_begin/end_file

tccgen_begin/end_file

This is supposed to make compilation and linking with
multiple source files (tcc f1.c f2.S ...) behave just
the same as linking object files.

tccgen.c:put_extern_sym2():
- use put_elf_sym to enter new symbols unconditionally

tccelf.c:
- save section state before compilation
- disable symbol hashing during compilation
- merge symbols and update relocations after compilation

tccpe.c:
- re-create s1->uw_sym for each compilation (because it
  may change)