runtime, syscall: switch linux/386 to use int 0x80

runtime, syscall: switch linux/386 to use int 0x80

Like bionic, musl also doesn't provide vsyscall helper in %gs:0x10,
and as int $0x80 is as fast as calling %gs:0x10, just use int $0x80
always.

Because we're no longer using vsyscall in VDSO, get rid of VDSO code
for linux/386 too.

Fixes #14476.

Change-Id: I00ec8652060700e0a3c9b524bfe3c16a810263f6
Reviewed-on: https://go-review.googlesource.com/19833
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/22038
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>