add gettid function

this is a prerequisite for addition of other interfaces that use
kernel tids, including futex and SIGEV_THREAD_ID.

there is some ambiguity as to whether the semantic return type should
be int or pid_t. either way, futex API imposes a contract that the
values fit in int (excluding some upper reserved bits). glibc used
pid_t, so in the interest of not having gratuitous mismatch (the
underlying types are the same anyway), pid_t is used here as well.

while conceptually this is a syscall, the copy stored in the thread
structure is always valid in all contexts where it's valid to call
libc functions, so it's used to avoid the syscall.

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