New proc supports_get_siginfo_type

New proc supports_get_siginfo_type

Both siginfo-obj.exp and siginfo-thread.exp have the same code
checking the support of geting a type of siginfo for a given arch.
This patch is to move these code into a proc supports_get_siginfo_type.

gdb/testsuite:

2015-06-24  Yao Qi  <yao.qi@linaro.org>

	* lib/gdb.exp (supports_get_siginfo_type): New proc.
	* gdb.base/siginfo-obj.exp: Invoke supports_get_siginfo_type.
	* gdb.base/siginfo-thread.exp: Likewise.