cmsgpack: Pass LDFLAGS to ld without "-Wl,"

Here, ld is called directly instead of gcc, while LDFLAGS is meant for
gcc's command line. LDFLAGS likely contains options that look like
"-Wl,O1", but ld recognizes "-O1", not "-Wl,-O1", so src_compile fails
if "-Wl," isn't stripped.

Change-Id: I8973f3cd3080a585c78120db1489f7c1938840f1