Cast to bfd_vma in arm-tdep.c

Cast to bfd_vma in arm-tdep.c

Some arm-tdep.c data structures use a bfd_vma.  A couple of spots will
warn about an implicit narrowing cast when building a gdb where
CORE_ADDR is 64-bit but bfd_vma is 32-bit.

This patch silences these warnings by changing the types in question
to CORE_ADDR.

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

	* arm-tdep.c (struct arm_mapping_symbol) <value>: Now a
	CORE_ADDR.
	(struct arm_exidx_entry) <addr>: Now a CORE_ADDR.