Add documentation to gdb_compile

Add documentation to gdb_compile

This patch adds some documentation to gdb_compile.  It describes the
various options that can influence compilation.  Most of them are
handled by DejaGnu, but are not really documented anywhere, so I think
it's good to have a quick reference.  Not all possible options are
described, that would add way to much noise.  I chose those that I think
are relevant in the context of writing a test case.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_compile): Add function doc.