testsuite: Fix missing EFFECTIVE_TARGETS variable errors

Permit running vector tests outside `check_vect_support_and_set_flags'
environment, removing errors such as:

ERROR: gcc.dg/analyzer/torture/pr93350.c   -O0 : can't read "EFFECTIVE_TARGETS": no such variable for " dg-require-effective-target 1 vect_int "

or:

ERROR: gcc.dg/bic-bitmask-13.c: error executing dg-final: can't read "EFFECTIVE_TARGETS": no such variable

with `mips-linux-gnu' target testing.

The EFFECTIVE_TARGETS variable has originated from commit 9b7937cf
("Add support to run auto-vectorization tests for multiple effective
targets."), where arrangements have been made to run vector tests run
within `check_vect_support_and_set_flags' environment iteratively over
all the vector unit variants available in the architecture using extra
compilation flags regardless of whether the target environment arranged
for a particular testsuite run has vector support enabled by default.
So far this has been used for the MIPS target only.

Vector tests have since been added though that run outside environment
set up by `check_vect_support_and_set_flags' just using the current
compilation environment with no extra flags added.  This works for most
targets, however causes problems with the MIPS target, because outside
`check_vect_support_and_set_flags' environment the EFFECTIVE_TARGETS
variable will not have been correctly set up even if it was added to
the particular script invoking the test in question.

Fix this by using just the current compilation environment whenever a
vector feature is requested by `et-is-effective-target' in the absence
of the EFFECTIVE_TARGETS variable.  This required some modification to
individual vector feature tests, which always added the compilation
flags required for the determination of whether the given vector unit
variant can be verified with the current testsuite run (except for the
Loongson MMI variant).  Now explicit flags are only passed in setting up
EFFECTIVE_TARGETS and otherwise the current compilation environment will
determine whether such a vector test is applicable.

This changes how Loongson MMI is handled in that the `-mloongson-mmi'
flag is explicitly passed for the determination of whether this vector
unit variant can be verified, which I gather is how it was supposed to
be arranged anyway because the flag is then added for testing the
Loongson MMI variant.

	gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_mpaired_single): Add `args' argument and
	pass it to `check_no_compiler_messages' replacing
	`-mpaired-single'.
	(add_options_for_mips_loongson_mmi): Add `args' argument and
	pass it to `check_no_compiler_messages'.
	(check_effective_target_mips_msa): Add `args' argument and pass
	it to `check_no_compiler_messages' replacing `-mmsa'.
	(check_effective_target_mpaired_single_runtime)
	(add_options_for_mpaired_single): Pass `-mpaired-single' to
	`check_effective_target_mpaired_single'.
	(check_effective_target_mips_loongson_mmi_runtime)
	(add_options_for_mips_loongson_mmi): Pass `-mloongson-mmi' to
	`check_effective_target_mips_loongson_mmi'.
	(check_effective_target_mips_msa_runtime)
	(add_options_for_mips_msa): Pass `-mmsa' to
	`check_effective_target_mips_msa'.
	(et-is-effective-target): Verify that EFFECTIVE_TARGETS exists
	and if not, just check if the current compilation environment
	supports the target feature requested.
	(check_vect_support_and_set_flags): Pass `-mpaired-single',
	`-mloongson-mmi', and `-mmsa' to the respective target feature
	checks.