Remove extendedPC.hpp and fetch_frame_from_ucontext

This is an old interface left over from *before* Safepoint polling and is an empty wrapper that gets included in 910 files:

#total # includes # lines
   39130 910 43 /jdk2/ken/open/src/hotspot/share/runtime/extendedPC.hpp

One of the uses of ExtendedPC is in a function:
-address os::Linux::fetch_frame_from_ucontext(Thread* thread,
- const ucontext_t* uc, intptr_t** ret_sp, intptr_t** ret_fp) {
-
- assert(thread != NULL, "just checking");
- assert(ret_sp != NULL, "just checking");
- assert(ret_fp != NULL, "just checking");
-
- return os::fetch_frame_from_context(uc, ret_sp, ret_fp);
-}

Which on all platforms just calls fetch_frame_from_context() so this should be removed also. This function used to do more on solaris/sparc:

ucontext_t *luc = os::Solaris::get_valid_uc_in_signal_handler(thread, uc);

This should be removed too, conveniently with this RFE.