luajit: bug fix - prevent target CFLAGS from being used in host tool compile.

The luajit amalgamation compile starts with building a host tool, and then
uses it to build itself.  However, when CFLAGS is specified, as opposed to
TARGET_CFLAGS, then it is used for both HOST and TARGET builds.  So if you
add something target specific into 'Target Optimizations' (for example,
'-mfpu=neon -mfloat-abi=softfp' for ARM Cortex-A8), then it gets into the
host tool compile, which then fails (because my build machine is not an
ARM Cortext-A8).  This can be fixed by using TARGET_CFLAGS and
TARGET_LDFLAGS, instead of CFLAGS and LDFLAGS, respectively.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>