libc: Make RLIMIT_STACK 1 MB in size

Currently, RLIMIT_STACK is 64 KB.  This is too small for the JVM which
limits stack size to RLIMIT_STACK if a VMA range that covers address
found in '__libc_stack_end' in /proc/self/maps.

So in preparation for procfs support, bump up RLIMIT_STACK to 1 MB.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>