Don't set breakpoints on import stubs on Windows amd64

Don't set breakpoints on import stubs on Windows amd64

On Windows amd64, setting a breakpoint on a symbol imported from a
shared library after that library is loaded creates a breakpoint with
two locations, one on the import stub, and another in the shared
library, while on i386, the breakpoint is only set in the shared
library.

This is due to the minimal symbol for the import stub not being
correctly given the type mst_solib_trampoline on Windows amd64, unlike
Windows i386.

As currently written, coff_symfile_read is always skipping over the
character after the "__imp_" (amd64) or "_imp_" (i386) prefix,
assuming that it is '_'.  However, while i386 is an underscored
target, amd64 is not.

On x86_64-pc-cygwin, it fixes:

 - FAIL: gdb.base/solib-symbol.exp: foo in libmd
 + PASS: gdb.base/solib-symbol.exp: foo in libmd

Unfortunately, several other tests which passed now fail but that's
because this issue was masking other problems.

No change on i686-pc-cygwin.

gdb/ChangeLog:
2015-03-26  Pedro Alves  <palves@redhat.com>
	    Jon TURNEY  <jon.turney@dronecode.org.uk>

	* coffread.c (coff_symfile_read): When constructing the name of an
	import stub symbol from import symbol for amd64, only skip the
	char after _imp_ if the target is underscored (like i386) and the
	char is indeed the target's leading char.