CFG_CFLAGS is only used for jemalloc, rename all uses to CFG_JEMALLOC_CFLAGS

i686-apple-ios already used CFG_JEMALLOC_CFLAGS, so merge that one
mk/rt/jemalloc: pass CFG_GCCISH_CFLAGS inside CC instead of passing CFG_CFLAGS in EXTRA_CFLAGS

 - CFG_CFLAGS is gone (it was previously only used by jemalloc anyhow).
 - CFG_JEMALLOC_CFLAGS may contain flags needed for the compiler to
   function (produce a binary output).
 - jemalloc's configure runs $(CC) without EXTRA_CFLAGS, and (without
   this change) will fail if any flags are required for CC to work.