IBM Z: fix `section type conflict` with -mindirect-branch-table

IBM Z: fix `section type conflict` with -mindirect-branch-table

s390_code_end () puts indirect branch tables into separate sections and
tries to switch back to wherever it was in the beginning by calling
switch_to_section (current_function_section ()).

First of all, this is unnecessary - the other backends don't do it.

Furthermore, at this time there is no current function, but if the
last processed function was cold, in_cold_section_p remains set.  This
causes targetm.asm_out.function_section () to call
targetm.section_type_flags (), which in absence of current function
decl classifies the section as SECTION_WRITE.  This causes a section
type conflict with the existing SECTION_CODE.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_code_end): Do not switch back to
	code section.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/nobp-section-type-conflict.c: New test.