builtin: add a 'v hash:' line to V panics

vlib/builtin/builtin.c.v

@@ -9,6 +9,10 @@ pub fn exit(code int) {
 	C.exit(code)
 }
 
+fn vcommithash() string {
+	return unsafe { tos5(&char(C.V_CURRENT_COMMIT_HASH)) }
+}
+
 // panic_debug private function that V uses for panics, -cg/-g is passed
 // recent versions of tcc print nicer backtraces automatically
 // NB: the duplication here is because tcc_backtrace should be called directly
@@ -24,6 +28,7 @@ fn panic_debug(line_no int, file string, mod string, fn_name string, s string) {
 		eprintln(' function: ${fn_name}()')
 		eprintln('  message: $s')
 		eprintln('     file: $file:$line_no')
+		eprintln('   v hash: $vcommithash()')
 		eprintln('=========================================')
 	} $else {
 		eprint('V panic: ')
@@ -63,6 +68,7 @@ pub fn panic_optional_not_set(s string) {
 pub fn panic(s string) {
 	eprint('V panic: ')
 	eprintln(s)
+	eprintln('v hash: $vcommithash()')
 	$if exit_after_panic_message ? {
 		C.exit(1)
 	} $else {