bitfields: one more hack and a "-Wgcc-compat" switch

bitfields: one more hack and a "-Wgcc-compat" switch

    bit_pos + bit_size > type_size * 8

must NEVER happen because the code generator can read/write
only the basic integral types.

Warn if tcc has to break GCC compatibility for that reason
and if -Wgcc-compat is given.

Example:
    struct __attribute__((packed)) _s
    {
        unsigned int x  : 12;
        unsigned char y :  7;
        unsigned int z  : 28;
    };

Expected (GCC) layout (sizeof struct = 6)
.xxxxxxxx.xxxxyyyy.yyyzzzzz.zzzzzzzz.zzzzzzzz.zzzzzzz0.

But we cannot read/write 'char y'from 2 bytes in memory.
So we have to adjust:
.xxxxxxxx.xxxx0000.yyyyyyyz.zzzzzzzz.zzzzzzzz.zzzzzzzz.zzz00000

Now 'int z' cannot be accessed from 5 bytes.  So we arrive
at this (sizeof struct = 7):
.xxxxxxxx.xxxx0000.yyyyyyy0.zzzzzzzz.zzzzzzzz.zzzzzzzz.zzzz0000

Otherwise the bitfield load/store generator needs to be
changed to allow byte-wise accesses.

Also we may touch memory past the struct in some cases
currently.  The patch adds a warning for that too.

   0:	55                   	push   %ebp
   1:	89 e5                	mov    %esp,%ebp
   3:	81 ec 04 00 00 00    	sub    $0x4,%esp
   9:	90                   	nop

    struct __attribute__((packed)) { unsigned x : 5; } b = {0} ;

   a:	8b 45 ff             	mov    -0x1(%ebp),%eax
   d:	83 e0 e0             	and    $0xffffffe0,%eax
  10:	89 45 ff             	mov    %eax,-0x1(%ebp)

This touches -0x1 ... +0x3(%ebp), hence 3 bytes beyond
stack space.  Since the data is not changed, nothing
else happens here.