PR breakpoints/21886: mem-break: Fix breakpoint insertion location

PR breakpoints/21886: mem-break: Fix breakpoint insertion location

Fix a commit cd6c3b4ffc4e ("New gdbarch methods breakpoint_kind_from_pc
and sw_breakpoint_from_kind") regression and restore the use of
`->placed_address' rather than `->reqstd_address' as the location for a
memory breakpoint to be inserted at.  Previously
`gdbarch_breakpoint_from_pc' was used that made that adjustment in
`default_memory_insert_breakpoint' from the preinitialized value,
however with the said commit that call is gone, so the passed
`->placed_address' has to be used for the initialization.

The regression manifests itself as the inability to debug any MIPS/Linux
compressed ISA dynamic executable as GDB corrupts the dynamic loader
with one of its implicit breakpoints, causing the program to crash, as
seen for example with the `mips-linux-gnu' target, o32 ABI, MIPS16 code,
and the gdb.base/advance.exp test case:

(gdb) continue
Continuing.

Program received signal SIGBUS, Bus error.
_dl_debug_initialize (ldbase=0, ns=0) at dl-debug.c:51
51	    r = &_r_debug;
(gdb) FAIL: gdb.base/advance.exp: Can't run to main

	gdb/
	PR breakpoints/21886
	* mem-break.c (default_memory_insert_breakpoint): Use
	`->placed_address' rather than `->reqstd_address' for the
	breakpoint location.