cmd/cgo: record CGO_LDFLAGS env var in _cgo_flags

cgo stores cgo LDFLAGS in _cgo_flags and _cgo_defun.c.
The _cgo_defun.c records the flags via
"#pragma cgo_ldflag <flag>", which external linking
relies upon for passing libraries (and search paths)
to the host linker.

The go command will allow LDFLAGS for cgo to be passed
through the environment (CGO_LDFLAGS); cgo ignores
this environment variable, and so its value doesn't
make it into the above mentioned files. This CL changes
cgo to record CGO_LDFLAGS also.

Fixes issue 5205.

R=iant, minux.ma
CC=golang-dev
https://codereview.appspot.com/8465043

Committer: Shenghou Ma <minux.ma@gmail.com>