samples/userspace: Set common malloc arena size to zero for syscall_perf

The default malloc arena when using picolibc is too large for the
hifive1_revb target.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

samples/userspace/syscall_perf/prj.conf

@@ -4,3 +4,4 @@ CONFIG_ASSERT=y
 CONFIG_LOG=y
 CONFIG_LOG_MODE_MINIMAL=y
 CONFIG_MAIN_STACK_SIZE=2048
+CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0