Add stackman_block_call for a malloc block - #28

stackman_block_call is a static inline in stackman.h, next to the stack-pointer macros. It takes a malloc block (the low address and the byte count) and fills in the pointer to pass to stackman_call, plus how many bytes of the block lie toward near.

On a descending stack that pointer is the aligned high end. When the base and the size are already aligned, call_sp == base + size and extent == size. On an ascending stack the pointer is the aligned low end, and an aligned base gives call_sp == base. Bytes dropped to reach the alignment are not part of extent. A null base, a zero size, a missing out-pointer, a range that would wrap, or a block that alignment leaves empty returns 0 and writes nothing.

Callers of stackman_call already do this conversion by hand, and the direction test is the part that gets copied wrong. The helper keeps that test in one place. A stack described as a far pointer and an extent stays in those terms; this function is only the step from a malloc pair to that pointer.

No version bump. Native make test on sysv_amd64 passed, including test_06 for the descending case and bin/test_block_asc, which compiles the function with STACKMAN_STACK_DIR set to 1. No supported ABI grows upward.