Breakpoints, don't skip prologue of ifunc resolvers with debug info

In v2:

  - The minsym/symbol matching in convert_linespec_to_sals is done
    differently.  The v1 bsearch method wouldn't work for PPC64, given
    function descriptors.  convert_linespec_to_sals will be touched
    again later in the series, for PPC64.
  - Moved declarations of locals closer to uses, because at some point
    I managed to reuse 'i' in an inner loop messing the outer loop...

Without this patch, some of the tests added to gdb.base/gnu-ifunc.exp
by a following patch fail like so:

  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=0: set-break: before resolving: break gnu_ifunc
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=0: set-break: before resolving: info breakpoints
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=0: set-break: after resolving: break gnu_ifunc
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=0: set-break: after resolving: info breakpoints
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=1: set-break: before resolving: break gnu_ifunc
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=1: set-break: before resolving: info breakpoints
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=1: set-break: after resolving: break gnu_ifunc
  FAIL: gdb.base/gnu-ifunc.exp: resolver_attr=0: resolver_debug=1: resolved_debug=1: set-break: after resolving: info breakpoints

All of them trigger iff:

 - you have debug info for the ifunc resolver.
 - the resolver and the user-visible symbol have the same name.

If you have an ifunc that has a resolver with the same name as the
user visible symbol, debug info for the resolver masks out the ifunc
minsym.  When you set a breakpoint by name on the user visible symbol,
GDB finds the DWARF symbol for the resolver, and thinking that it's a
regular function, sets a breakpoint location past its prologue.

Like so, location 1.2, before the ifunc is resolved by the inferior:

  (gdb) break gnu_ifunc
  Breakpoint 2 at 0x7ffff7bd36ea (2 locations)
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  1       breakpoint     keep y   <MULTIPLE>
  1.1                         y     0x00007ffff7bd36ea <gnu_ifunc>
  1.2                         y     0x00007ffff7bd36f2 in gnu_ifunc at src/gdb/testsuite/gdb.base/gnu-ifunc-lib.c:34
  (gdb)

And like so, location 2.2, if you set the breakpoint after the ifunc
is resolved by the inferior (to "final"):

  (gdb) break gnu_ifunc
  Breakpoint 5 at 0x400757 (2 locations)
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   <MULTIPLE>
  2.1                         y     0x000000000040075a in final at src/gdb/testsuite/gdb.base/gnu-ifunc-resd.c:21
  2.2                         y     0x00007ffff7bd36f2 in gnu_ifunc at src/gdb/testsuite/gdb.base/gnu-ifunc-lib.c:34
  (gdb)

I don't think this is right because when users set a breakpoint at an
ifunc, they don't care about debugging the resolver.  Instead what you
should is a single location for the ifunc in the first case, and a
single location of the ifunc target in the second case.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* linespec.c (struct bound_minimal_symbol_search_key): New.
	(convert_linespec_to_sals): Sort minimal symbols earlier.  Don't
	skip first line if we found a GNU ifunc minimal symbol by name.
	(compare_msymbols): Change parameters to work with a destructured
	lhs minsym.
	(compare_msymbols_for_qsort, compare_msymbols_for_bsearch): New
	functions.