PR11778 Add sprint_stack and sprint_ustack tapset functions.

PR11778 Add sprint_stack and sprint_ustack tapset functions.

New tapset functions that take a hexstring and produce a (simplified) stack
as string. Simplified because otherwise it would quickly run over max string
length.

* runtime/sym.h (_STP_SYM_SIMPLE): New format that gives only symbol,
  module, offset (and newline).
* tapset/context-symbols.stp (sprint_stack): New tapset function.
* tapset/ucontext-symbols.stp (sprint_ustack): Likewise.
* testsuite/buildok/context-symbols-embedded.stp: New sprint_stack test.
* testsuite/buildok/ucontext-symbols-embedded.stp: New sprint_ustack test.