Check return value of mmap(2) correctly and set correct flags. Now
mmap(2) actually returns non-stack memory and triggers a SIGSEGV
due to MAP_STACK on i386.

Add lastcomm(1) test for system call from writeable memory. This
requires that /usr/obj is mounted wxallowed, otherwise the test is
skipped. Split the stack map test into two stack pointer validation
tests, one during syscall and one in the trap handler.
Currently the tests during system call fail on amd64 as the signal
handler is not executed properly. On i386 and arm64 they pass. On
armv7 something else is broken.

Add lastcomm(1) test for system call from writeable memory. This
requires that /usr/obj is mounted wxallowed, otherwise the test is
skipped. Split the stack map test into two stack pointer validation
tests, one during syscall and one in the trap handler.
Currently the tests during system call fail on amd64 as the signal
handler is not executed properly. On i386 and arm64 they pass. On
armv7 something else is broken.