runtime: rename Lock to Mutex

Mutex is consistent with package sync, and when in the
unexported Go form it avoids having a conflcit between
the type (now mutex) and the function (lock).

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, r
https://codereview.appspot.com/133140043
cmd/api: fix for Lock⇒Mutex change

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/133150043
cmd/cc: translate C Eface type as Go interface{}

This will allow structs containing Efaces in C to be
manipulated as structs containing real interfaces in Go.
The eface struct is still defined for use by Go code.

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, khr, r
https://codereview.appspot.com/133980044
runtime: fix nacl/386 build

I changed all the NACL_SYSJMP to NACL_SYSCALL in
an earlier CL, but I missed the fact that NACL_SYSCALL
will push another return PC on the stack, so that the
arguments will no longer be in the right place.
Since we have to make our own call, we also have to
copy the arguments. Do that.

Fixes nacl/386 build.

TBR=minux
CC=golang-codereviews
https://codereview.appspot.com/135050044