[binutils][gas] Fix build failure with -std=c89

My previous patch introduced the use of a C99 feature.  C99 standard is
not required for gas, so this feature should be removed.

Committed as obvious.

gas/ChangeLog:

2020-02-11  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/tc-arm.c (vcx_handle_register_arguments): Remove `for`
	loop initial declaration.