arm-tdep: replace arm_mapping_symbol VEC with std::vector

arm-tdep: replace arm_mapping_symbol VEC with std::vector

This patch replaces VEC (arm_mapping_symbol) with an std::vector.  No
functional changes intended.

gdb/ChangeLog:

	* arm-tdep.c (struct arm_mapping_symbol) (operator <): New.
	(arm_mapping_symbol_s): Remove.
	(DEF_VEC_O(arm_mapping_symbol_s)): Remove.
	(arm_mapping_symbol_vec): New typedef.
	(struct arm_per_objfile): Add constructor.
	<section_maps>: Change type to
	std::unique_ptr<arm_mapping_symbol_vec[]>.
	(arm_compare_mapping_symbols): Remove.
	(arm_find_mapping_symbol): Adjust to section_maps type change.
	(arm_objfile_data_free): Call delete on arm_per_objfile.
	(arm_record_special_symbol): Adjust to section_maps type change.
	Allocate arm_per_objfile with new.