runtime: drop _SigUnblock for SIGSYS on Linux

The _SigUnblock flag was appended to SIGSYS slot of runtime signal table
for Linux in https://go-review.googlesource.com/22202, but there is
still no concrete opinion on whether SIGSYS must be an unblocked signal
for runtime.

This change removes _SigUnblock flag from SIGSYS on Linux for
consistency in runtime signal handling and adds a reference to #15204 to
runtime signal table for FreeBSD.

Updates #15204.

Change-Id: I42992b1d852c2ab5dd37d6dbb481dba46929f665
Reviewed-on: https://go-review.googlesource.com/22537Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_freebsd.go

@@ -24,7 +24,7 @@ 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 */ {_SigNotify, "SIGSYS: bad system call"}, // see golang.org/issues/15204
 	/* 13 */ {_SigNotify, "SIGPIPE: write to broken pipe"},
 	/* 14 */ {_SigNotify, "SIGALRM: alarm clock"},
 	/* 15 */ {_SigNotify + _SigKill, "SIGTERM: termination"},

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 */ {_SigThrow + _SigUnblock, "SIGSYS: bad system call"},
+	/* 31 */ {_SigThrow, "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 */ {_SigThrow + _SigUnblock, "SIGSYS: bad system call"},
+	/* 12 */ {_SigThrow, "SIGSYS: bad system call"},
 	/* 13 */ {_SigNotify, "SIGPIPE: write to broken pipe"},
 	/* 14 */ {_SigNotify, "SIGALRM: alarm clock"},
 	/* 15 */ {_SigNotify + _SigKill, "SIGTERM: termination"},