Fix CORE_ADDR size assertion in symfile-mem.c

Fix CORE_ADDR size assertion in symfile-mem.c

symfile-mem.c has some assertions about the size of various types, to
ensure that gdb and BFD don't get out of sync in a way that would
cause bugs.

Once CORE_ADDR is always 64-bit, one of these assertions can fail for
a 32-bit BFD build.  However, the real requirement here is just that
CORE_ADDR is wider -- because this code promotes a bfd_vma to a
CORE_ADDR.

This patch corrects the assert.

gdb/ChangeLog
2020-03-12  Tom Tromey  <tom@tromey.com>

	* symfile-mem.c: Update CORE_ADDR size assert.