Fix testsuite regression by: Do not skip prologue for asm (.S) files

Fix testsuite regression by: Do not skip prologue for asm (.S) files

I have somehow missed gdb.asm/asm-source.exp PASS->FAIL even on x86_64.

It has no longer valid assumption that "break" breaks after the prologue even
in assembler.  So I have changed this assumption of the testfile.

gdb/testsuite/ChangeLog
2015-07-10  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.asm/asm-source.exp (f at main): Stop at gdbasm_enter.
	(n at main): New.
	* gdb.asm/asmsrc1.s: Add comment "mark: main enter".