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 #5205.
R=iant, minux.ma
CC=golang-dev
https://golang.org/cl/
8465043