Fix unrecognised -mcpu target 'armv7-a' on arm-wrs-vxworks7

In the removal of arm-wrs-vxworks, the default cpu was updated from arm8
to armv7-a, but this is not recognized as a valid -mcpu target.  There
is however generic-armv7-a, which was likely the intended cpu that
should have been used instead.

gcc/ChangeLog:

	PR target/95420
	* config.gcc (arm-wrs-vxworks7*): Set default cpu to generic-armv7-a.

(cherry picked from commit 86b14bb1a93d8ef01678d132013a2006a4f00deb)

gcc/config.gcc

@@ -1192,7 +1192,7 @@ arm-wrs-vxworks7*)
 	tmake_file="${tmake_file} arm/t-arm arm/t-vxworks arm/t-bpabi"
 	tm_file="elfos.h arm/elf.h arm/bpabi.h arm/aout.h ${tm_file}"
 	tm_file="${tm_file} vx-common.h vxworks.h arm/vxworks.h"
-	target_cpu_cname="armv7-a"
+	target_cpu_cname="generic-armv7-a"
 	need_64bit_hwint=yes
 	;;
 arm*-*-freebsd*)                # ARM FreeBSD EABI