cmd/cgo: fix handling of defs_linux.go
Instead of including <sys/types.h> to get size_t, instead include
the ISO C standard <stddef.h> header, which defines fewer additional
types at risk of colliding with the user code. In particular, this
prevents collisions between <sys/types.h>'s userspace definitions with
the kernel definitions needed by defs_linux.go.
Also, -cdefs mode uses #pragma pack, so we can keep misaligned fields.
Fixes #8477.
LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/
120610043