libzstd: Don't warn about stack frame size in userspace

With the current way CFLAGS are modified in libzstd, CFLAGS passed on
the make command line will cause the CFLAGS in the Makefile for zstd.c
to be discarded, but not AM_CFLAGS. This causes a smaller frame size
limit to be used, and the build fails.

We don't need to worry about stack frame sizes in userspace. Drop the
extra flags.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10773