gdb: use foreach_with_prefix in gdb.base/break-interp.exp

gdb: use foreach_with_prefix in gdb.base/break-interp.exp

Use foreach_with_prefix, instead of foreach and with_test_prefix
separately.  Since allows removing some indentation levels, and formats
the test names a bit nicer, in my opinion (or at least, it's more
consistent with the rest of the testsuite):

    - PASS: gdb.base/break-interp.exp: LDprelinkNOdebugNO: BINprelinkNOdebugNOpieNO: INNER: core: set verbose on
    + PASS: gdb.base/break-interp.exp: ldprelink=NO: ldsepdebug=NO: binprelink=NO: binsepdebug=NO: binpie=NO: INNER: set verbose on

Note: this patch is better viewed with "git show -w" to ignore
whitespace changes.

gdb/testsuite/ChangeLog:

	* gdb.base/break-interp.exp: Use foreach_with_prefix.