runtime: updated SIGSYS to cause a panic + stacktrace

On GNU/Linux, SIGSYS is specified to cause the process to terminate
without a core dump. In https://codereview.appspot.com/3749041 , it
appears that Golang accidentally introduced incorrect behavior for
this signal, which caused Golang processes to keep running after
receiving SIGSYS. This change reverts it to the old/correct behavior.

Updates #15204

Change-Id: I3aa48a9499c1bc36fa5d3f40c088fdd7599e0db5
Reviewed-on: https://go-review.googlesource.com/22202Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sigtab_linux_generic.go

@@ -45,7 +45,7 @@ var sigtable = [...]sigTabT{
 	/* 28 */ {_SigNotify, "SIGWINCH: window size change"},
 	/* 29 */ {_SigNotify, "SIGIO: i/o now possible"},
 	/* 30 */ {_SigNotify, "SIGPWR: power failure restart"},
-	/* 31 */ {_SigNotify, "SIGSYS: bad system call"},
+	/* 31 */ {_SigThrow + _SigUnblock, "SIGSYS: bad system call"},
 	/* 32 */ {_SigSetStack + _SigUnblock, "signal 32"}, /* SIGCANCEL; see issue 6997 */
 	/* 33 */ {_SigSetStack + _SigUnblock, "signal 33"}, /* SIGSETXID; see issues 3871, 9400, 12498 */
 	/* 34 */ {_SigNotify, "signal 34"},

src/runtime/sigtab_linux_mips64x.go

@@ -25,7 +25,7 @@ var sigtable = [...]sigTabT{
 	/* 9 */ {0, "SIGKILL: kill"},
 	/* 10 */ {_SigPanic + _SigUnblock, "SIGBUS: bus error"},
 	/* 11 */ {_SigPanic + _SigUnblock, "SIGSEGV: segmentation violation"},
-	/* 12 */ {_SigNotify, "SIGSYS: bad system call"},
+	/* 12 */ {_SigThrow + _SigUnblock, "SIGSYS: bad system call"},
 	/* 13 */ {_SigNotify, "SIGPIPE: write to broken pipe"},
 	/* 14 */ {_SigNotify, "SIGALRM: alarm clock"},
 	/* 15 */ {_SigNotify + _SigKill, "SIGTERM: termination"},