Fix a problem with the libgomp testsuite using a method to determine
the compiler to use resulting in the tool being different from one the
library has been built with, and causing a catastrophic failure from the
lack of a suitable `--sysroot=' option where the `--with-build-sysroot='
configuration option has been used to build the compiler resulting in
the inability to link executables.

Address this problem by defining the compiler to use, via the
GCC_UNDER_TEST TCL variable, set in the DejaGNU configuration file from
$CC by autoconf, which will have all the required options set for the
target compiler to build executables in the environment configured,
removing failures like:

.../bin/riscv64-linux-gnu-ld: cannot find crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lm
.../bin/riscv64-linux-gnu-ld: cannot find -lpthread
.../bin/riscv64-linux-gnu-ld: cannot find -lc
.../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 crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lm
.../bin/riscv64-linux-gnu-ld: cannot find -lpthread
.../bin/riscv64-linux-gnu-ld: cannot find -lc
.../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		44
# of unexpected failures	3274
# of unresolved testcases	3241
# 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/libgomp-test-support.exp.in (GCC_UNDER_TEST): New
	variable.

From-SVN: r279708