implement test coverage

I have implemented the -ftest-coverage option. It works a bit different
from the gcc version. It output .tcov text file which looks almost the
same as a gcov file after a executable/so file is run.

Add lib/tcov.c file
Modify Makefiles to compile/install it
Add -ftest-coverage option in tcc.c/tcc.h/tcc-doc.texi
Add code to tccelf.c/tccgen.c/tccpe.c
Add gen_increment_tcov to tcc.h/*gen.c

unrelated changes:
Add sigemptyset in tccrun.c
Fix riscv64-gen.c tok_alloc label size

i386-gen.c

@@ -1023,6 +1023,19 @@ ST_FUNC void gen_cvt_csti(int t)
         );
 }
 
+/* increment tcov counter */
+ST_FUNC void gen_increment_tcov (SValue *sv)
+{
+   o(0x0583); /* addl $1, xxx */
+   greloc(cur_text_section, sv->sym, ind, R_386_32);
+   gen_le32(0);
+   o(1);
+   o(0x1583); /* addcl $0, xxx */
+   greloc(cur_text_section, sv->sym, ind, R_386_32);
+   gen_le32(4);
+   g(0);
+}
+
 /* computed goto support */
 ST_FUNC void ggoto(void)
 {