[ARM] Fix warning cannot find thumb start symbol

[ARM] Fix warning cannot find thumb start symbol

In case params.thumb_entry_symbol has its default NULL value, using it
to print a warning in gld${EMULATION_NAME}_finish results in a crash.

Use h->root.string instead which either points to
params.thumb_entry_symbol name or to entry_symbol name.

2018-07-09  Christophe Lyon  <christophe.lyon@linaro.org>

	* emultempl/armelf.em (gld${EMULATION_NAME}_finish): Use the right
	symbol name in case of warning.

Change-Id: Iec61a833c0ad538b1440bf326ba67834c314dd63