runtime: rename Sigaltstack to SigaltstackT

Avoids a conflict between the type and function sigaltstack.

LGTM=crawshaw
R=rsc, crawshaw
CC=golang-codereviews
https://golang.org/cl/138920043
diff --git a/src/pkg/runtime/defs1_linux.go b/src/pkg/runtime/defs1_linux.go
index 451817a..392cc4a 100644
--- a/src/pkg/runtime/defs1_linux.go
+++ b/src/pkg/runtime/defs1_linux.go
@@ -31,7 +31,7 @@
 type Xmmreg1 C.struct__xmmreg
 type Fpstate1 C.struct__fpstate
 type Fpreg1 C.struct__fpreg
-type Sigaltstack C.struct_sigaltstack
+type SigaltstackT C.struct_sigaltstack
 type Mcontext C.mcontext_t
 type Ucontext C.ucontext_t
 type Sigcontext C.struct_sigcontext