libc - Use sbrk() system call, remove brk()
* Attempt to use the kernel sbrk() system call. If it fails
with EOPNOTSUPP, back-off to the older libc sbrk() emulation
code which uses still-present but obsolete break() system call.
* Changes support both old and new kernels. Older kernels do not
implement the sbrk() system call, causing the call to return
-1/EOPNOTSUPP. Note that older kernels did not generate an
ENOSYS/signal for sbrk().
* sbrk() cannot support negative increments. Allow only 0 and positive
increments.
* Fix MPSAFE issues (requires new kernels) while we are here.
* Remove brk() support entirely. This was also emulated in older
kernels using break().
* These changes are necessary in order to support mmap() hinting above
RLIMIT_DATA's current value. Older kernels only support hinting above
the system maximum value (above the 32GB mark). Several languages in
dports really want to mmap() addresses below 4GB.
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