tcg-i386: Implement small forward branches.

tcg-i386: Implement small forward branches.

There are places, like brcond2, where we know that the destination
of a forward branch will be within 127 bytes.

Add the R_386_PC8 relocation type to support this.  Add a flag to
tcg_out_jxx and tcg_out_brcond* to enable it.  Set the flag in the
brcond2 label_next branches; pass along the input flag otherwise.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>