Move 'find-dg-do-what' from 'gcc/testsuite/lib/gcc-dg.exp' into 'gcc/testsuite/lib/target-supports-dg.exp'

This was added in commit f553b1aaa2b1b925c918e5dcf966290b045321c2
"Refactor duplicated code into 'gcc/testsuite/lib/gcc-dg.exp:find-dg-do-what'",
for use by 'gcc/testsuite/lib/target-supports.exp'.  The latter is used by
several test suites, 'gcc/testsuite/lib/gcc-dg.exp' however doesn't get loaded
for 'make check-target-libstdc++-v3', for example, and testing ERRORs out:

    ERROR: (DejaGnu) proc "find-dg-do-what" does not exist.
    The error code is TCL LOOKUP COMMAND find-dg-do-what
    The info on the error is:
    invalid command name "find-dg-do-what"
        while executing
    "::tcl_unknown find-dg-do-what"
        ("uplevel" body line 1)
        invoked from within
    "uplevel 1 ::tcl_unknown $args"

Fix this by moving 'find-dg-do-what' into the DejaGnu interfacing file
corresponding to 'gcc/testsuite/lib/target-supports.exp':
'gcc/testsuite/lib/target-supports-dg.exp' (next to other related procedures).

	gcc/testsuite/
	* lib/gcc-dg.exp (find-dg-do-what): Move...
	* lib/target-supports-dg.exp: ... here.
2 files changed