Remove CFG_CFLAGS in favor of CFG_JEMALLOC_FLAGS

- CFG_CFLAGS is only used for jemalloc
- We grew an extra set of flags for jemalloc (CFG_JEMALLOC_CFLAGS) in addition to CFG_CFLAGS
- This kills of CFG_CFLAGS and keeps the more specific and less confusing CFG_JEMALLOC_CFLAGS
- Additionally, pass CFG_JEMALLOC_CFLAGS to jemalloc's configure slightly differently so things work when people use --sysroot in their CFLAGS.