Fix __minimal_malloc segfaults in __mmap due to stack-protector

Starting with commit b05fae4d
"elf: Use the minimal malloc on tunables_strdup",
I get lots of segfaults in static tests on s390x when also using, e.g.:
export GLIBC_TUNABLES="glibc.elision.enable=1"

tunables_strdup callls __minimal_malloc which tries to call __mmap
due to insufficient space left. __mmap itself first setups a new
stack frame and segfaults when copying the stack-protector canary
from thread-pointer. The latter one is not yet setup.

Thus this patch also turns off stack-protection for mmap.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>