cmake: Turn off OS_RT_MALLOC_SBRK under VxWorks.
tlsf: Use sysconf(_SC_PAGESIZE) instead of getpagesize().

getpagesize() was in SUSv2 but was marked legacy, with the
recommendation to replace its usage with sysconf(_SC_PAGE_SIZE) or
sysconf(_SC_PAGESIZE).  It does not appear in POSIX:2001.

Also, sysconf(_SC_PAGESIZE) is available under VxWorks 6.9, while
getpagesize() is not.