tcc.h:
* cleanup struct 'Sym'
* include some 'Attributes' into 'Sym'
* in turn get rid of VT_IM/EXPORT, VT_WEAK
* re-number VT_XXX flags
* replace some 'long' function args by 'int'

tccgen.c:
* refactor parse_btype()

tccgen.c: remove any 'nocode_wanted' checks, except in
- greloca(), disables output elf symbols and relocs
- get_reg(), will return just the first suitable reg)
- save_regs(), will do nothing

Some minor adjustments were made where nocode_wanted is set.

xxx-gen.c: disable code output directly where it happens
in functions:
- g(), output disabled
- gjmp(), will do nothing
- gtst(), dto.

C standard specifies that array should be declared with a non null size
or with * for standard array. Declaration of relocs_info in tcc.h was
not respecting this rule. This commit add a R_NUM macro that maps to the
R_<ARCH>_NUM macros and declare relocs_info using it. This commit also
moves all linker-related macros from <arch>-gen.c files to <arch>-link.c
ones.

Factor most of common logic between targets in build_got_entries by
defining target specific info into structures in the backends.

This reverts commit 4e04f67c94c97b4f28a4d73759a0ad38fb3a10f7. Requested by grischka.

 * Documentation is now in "docs".
 * Source code is now in "src".
 * Misc. fixes here and there so that everything still works.

I think I got everything in this commit, but I only tested this
on Linux (Make) and Windows (CMake), so I might've messed
something up on other platforms...

Still some more tabs to be taken care of. arm-gen.c and tcccoff.c
have so many style issues that I'm just going to throw clang-format
at them.