optionrom: fix compilation with mingw docker target

Two fixes are needed. First, mingw does not have -D_FORTIFY_SOURCE,
hence --enable-debug disables optimization. This is not acceptable
for ROMs, which should override CFLAGS to force inclusion of -O2.

Second, PE stores global constructors and destructors using the
following linker script snippet:

___CTOR_LIST__ = .; __CTOR_LIST__ = . ;
LONG (-1);*(.ctors); *(.ctor); *(SORT(.ctors.*)); LONG (0);
___DTOR_LIST__ = .; __DTOR_LIST__ = . ;
LONG (-1); *(.dtors); *(.dtor); *(SORT(.dtors.*)); LONG (0);

The LONG directives cause the .img files to be 16 bytes too large;
the recently added check to signrom.py catches this. To fix this,
replace -T and -e options with a linker script.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>