Use standard_testfile in gdb.arch/thumb-prologue.exp and gdb.arch/thumb2-it.exp

Use standard_testfile in gdb.arch/thumb-prologue.exp and gdb.arch/thumb2-it.exp

This patch fixes the errors below:

Running /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.arch/thumb-prologue.exp ...
gdb compile failed, arm-linux-gnueabihf/bin/ld: cannot open output file /scratch/yao/gdb/build-git/arm-linux-gnueabihf/gdb/testsuite/gdb.arch/thumb-prologue: No such file or directory
collect2: error: ld returned 1 exit status
Running /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.arch/thumb2-it.exp ...
gdb compile failed, arm-linux-gnueabihf/bin/ld: cannot open output file /scratch/yao/gdb/build-git/arm-linux-gnueabihf/gdb/testsuite/gdb.arch/thumb2-it: No such file or directory

gdb/testsuite:

2016-05-23  Yao Qi  <yao.qi@linaro.org>

	* gdb.arch/thumb-prologue.exp: Use standard_testfile.
	* gdb.arch/thumb2-it.exp: Likewise.