Eliminate VT_ENUM as a basic type.  Instead use the integral
types VT_InT/VT_LLONG with an additional flag to indicate
that it is an enum.

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()

- configure:
  - add --config-uClibc,-musl switch and suggest to use
    it if uClibc/musl is detected
  - make warning options magic clang compatible
  - simplify (use $confvars instead of individual options)
- Revert "Remove some unused-parameter lint"
  7443db0d5f841b81a55e918bf8c228dd20f9ddb2
  rather use -Wno-unused-parameter (or just not -Wextra)
- #ifdef functions that are unused on some targets
- tccgen.c: use PTR_SIZE==8 instead of (X86_64 || ARM64)
- tccpe.c: fix some warnings
- integrate dummy arm-asm better

Also:
- on windows i386 and x86-64, structures of size <= 8 are
  NOT returned in registers if size is not one of 1,2,4,8.
- cleanup: put all tv-push/pop/swap/rot into one place

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.

VT_PTR needs to be handled like VT_LLONG.

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.

With -run the call instruction and a defined function can be
far away, if the function is defined in the executable itself,
not in the to be compiled code.  So we always need PLT slots
for -run, not just for undefined symbols.

This is a work-around for TCC's linker, on AArch64, not building a PLT
when TCC is invoked with "-run". Fixing the linker should be possible:
it works on arm and x86_64, apparently.