Fix register selection in var-access.exp

Fix register selection in var-access.exp

The new test var-access.exp causes FAILs on i686.  This is because the
test chooses the wrong name for DWARF register number 1: It uses
"edx" (which corresponds to DWARF register number 2), but should have used
"ecx" instead.

Also, the current logic in var-access.exp does not correctly distinguish
between a 64-bit and a 32-bit program on an x86-64 target.  It uses the
64-bit register names for both.

These problems are fixed.  In order to address the latter, the convenience
macros is_*_target are exploited where appropriate.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/var-access.exp: Use register name ecx instead of edx
	on 32-bit x86 targets.  Exploit is_*_target macros where
	appropriate.