Preserve syscall results during application invocation

[rrnewton]

dr_invoke_syscall_as_app() ran post_system_call() without first publishing the raw kernel return value in the saved machine context. Post-syscall handlers therefore observed the stale syscall-number register; for SYS_mmap on x86-64 this is 9 instead of the returned mapping address and can corrupt DynamoRIO memory bookkeeping or crash the guest.\n\nStore the raw return value before post_system_call(), then return the possibly adjusted result. Extend client.strace with an anonymous mmap/munmap invocation to exercise post-syscall memory bookkeeping.\n\nDownstream reproducer: a DynamoRIO client that forwards application mmap through dr_invoke_syscall_as_app caused /usr/bin/sort and /usr/bin/md5sum to SIGSEGV before guest output.