RISC-V: Enable -gvariable-location-views by default

RISC-V: Enable -gvariable-location-views by default

This affects only the RISC-V targets, where the compiler options
-gvariable-location-views and consequently also -ginline-points
are disabled by default, which is unexpected and disables some
useful features of the generated debug info.

Due to a bug in the gas assembler the .loc statement
is not usable to generate location view debug info.
That is detected by configure:

configure:31500: checking assembler for dwarf2 debug_view support
configure:31509: .../riscv-unknown-elf/bin/as    -o conftest.o conftest.s >&5
conftest.s: Assembler messages:
conftest.s:5: Error: .uleb128 only supports constant or subtract expressions
conftest.s:6: Error: .uleb128 only supports constant or subtract expressions
configure:31512: $? = 1
configure: failed program was
        .file 1 "conftest.s"
        .loc 1 3 0 view .LVU1
        nop
        .data
        .uleb128 .LVU1
        .uleb128 .LVU1

configure:31523: result: no

This results in dwarf2out_as_locview_support being set to false,
and that creates a sequence of events, with the end result that
most inlined functions either have no DW_AT_entry_pc, or one
with a wrong entry pc value.

But the location views can also be generated without using any
.loc statements, therefore we should enable the option
-gvariable-location-views by default, regardless of the status
of -gas-locview-support.

Note however, that the combination of the following compiler options
-g -O2 -gvariable-location-views -gno-as-loc-support
turned out to create invalid assembler intermediate files,
with lots of assembler errors like:
Error: leb128 operand is an undefined symbol: .LVU3

This affects all targets, except RISC-V of course ;-)
and is fixed by the changes in dwarf2out.cc

Finally the .debug_loclists created without assembler locview support
did contain location view pairs like v0000000ffffffff v000000000000000
which is the value from FORCE_RESET_NEXT_VIEW, but that is most likely
not as expected either, so change that as well.

gcc/ChangeLog:

	* dwarf2out.cc (dwarf2out_maybe_output_loclist_view_pair,
	output_loc_list): Correct handling of -gno-as-loc-support,
	use ZERO_VIEW_P to output view number as zero value.
	* toplev.cc (process_options): Do not automatically disable
	-gvariable-location-views when -gno-as-loc-support or
	-gno-as-locview-support is used, instead do automatically
	disable -gas-locview-support if -gno-as-loc-support is used.

gcc/testsuite/ChangeLog:

	* gcc.dg/debug/dwarf2/inline2.c: Add checks for inline entry_pc.
	* gcc.dg/debug/dwarf2/inline6.c: Add -gno-as-loc-support and check
	the resulting location views.