Force language to C in gdb.compile/compile-ifunc.exp

Force language to C in gdb.compile/compile-ifunc.exp

Due to the lack of debug information in the binary, GDB is unable to figure
out what language is being used.  This may be a problem when doing remote
debugging and the binary stops at the entry point containing asm code.

In this case GDB will switch to asm as current language and will not switch
back to C when it reaches main, which in turn causes the compile feature check
to malfunction.

This is solved by forcing the language to C after reaching main.

gdb/testsuite/ChangeLog:

2015-08-26  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.compile/compile-ifunc.exp (with_test_prefix): Force language
	to C.