BaseTools/GenMake: Use ToolDefinition as fallback option

BaseTools/GenMake: Use ToolDefinition as fallback option

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3653

Use the value set in tools_def.txt when the makefile type is
not explicitly set via BuildOption. This allows to have a
valid default makefile name instead of an empty string.

Also use GMAKE_FILETYPE instead of hard-coded "gmake".

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>