gdb: use std::vector instead of alloca in core_target::get_core_register_section

gdb: use std::vector instead of alloca in core_target::get_core_register_section

As I was trying to compile gdb for an m68k host, I got this error:

  CXX    corelow.o
In file included from /binutils-gdb/gdb/gdbsupport/common-defs.h:120,
                 from /binutils-gdb/gdb/defs.h:28,
                 from /binutils-gdb/gdb/corelow.c:20:
/binutils-gdb/gdb/corelow.c: In member function 'void core_target::get_core_register_section(regcache*, const regset*, const char*, int, int, const char*, bool)':
/binutils-gdb/gdb/../include/libiberty.h:727:36: error: 'alloca' bound is unknown [-Werror=alloca-larger-than=]
  727 | # define alloca(x) __builtin_alloca(x)
      |                    ~~~~~~~~~~~~~~~~^~~
/binutils-gdb/gdb/corelow.c:625:23: note: in expansion of macro 'alloca'
  625 |   contents = (char *) alloca (size);
      |                       ^~~~~~

We are using alloca to hold the contents of a the core register
sections.  These sections are typically fairly small, but there is no
realy guarantee, so I think it would be more reasonable to just use
dynamic allocation here.

gdb/ChangeLog:

	* corelow.c (core_target::get_core_register_section): Use
	  std::vector instead of alloca.