Add flags to find libatomic in build-tree testing, fixing a catastrophic
libgomp testsuite failure with `riscv*-*-linux*' targets, which imply
`-latomic' with the `-pthread' GCC option, implied in turn by the
`-fopenacc' and `-fopenmp' options, removing failures like:

.../bin/riscv64-linux-gnu-ld: cannot find -latomic
collect2: error: ld returned 1 exit status
compiler exited with status 1
FAIL: libgomp.c/../libgomp.c-c++-common/atomic-18.c (test for excess errors)
Excess errors:
.../bin/riscv64-linux-gnu-ld: cannot find -latomic

UNRESOLVED: libgomp.c/../libgomp.c-c++-common/atomic-18.c compilation failed to produce executable

and bringing overall test results for the `riscv64-linux-gnu' target
(here with the `x86_64-linux-gnu' host and RISC-V QEMU in the Linux user
emulation mode as the target board) from:

		=== libgomp Summary ===

# of expected passes		90
# of unexpected failures	3267
# of expected failures		2
# of unresolved testcases	3247
# of unsupported tests		548

to:

		=== libgomp Summary ===

# of expected passes		6834
# of unexpected failures	4
# of expected failures		4
# of unsupported tests		518

	libgomp/
	* testsuite/lib/libgomp.exp (libgomp_init): Add flags to find
	libatomic in build-tree testing.

From-SVN: r278505