asm-generic/sections.h: extend assembler push_section_type()

Add flags support to the assembler version of push_section_type().
This also makes then SECTION_TYPE() be the assembler default if
the flags are empty. This is optional, but since we are enabling

SECTION_TYPE() to be defined for all code this lets us keep a
definition for it for C code, assembly, and assembler.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>