jit: fix leak of pending_assemble_externals_set [PR117275]

jit: fix leak of pending_assemble_externals_set [PR117275]

My recent r15-4580-g779c0390e3b57d fix for resetting state in
varasm.cc introduced some noise to "make selftest-valgrind" and,
presumably, a memory leak in libgccjit:

==2462086== 160 (56 direct, 104 indirect) bytes in 1 blocks are definitely lost in loss record 248 of 352
==2462086==    at 0x5270E7D: operator new(unsigned long) (vg_replace_malloc.c:342)
==2462086==    by 0x1D1EB89: init_varasm_once() (varasm.cc:6806)
==2462086==    by 0x181C845: backend_init() (toplev.cc:1826)
==2462086==    by 0x181D41A: do_compile() (toplev.cc:2193)
==2462086==    by 0x181D99C: toplev::main(int, char**) (toplev.cc:2371)
==2462086==    by 0x378391D: main (main.cc:39)

Fixed thusly.

gcc/ChangeLog:
	PR jit/117275
	* varasm.cc (process_pending_assemble_externals): Reset
	pending_assemble_externals_set to nullptr after deleting it.
	(varasm_cc_finalize): Delete pending_assemble_externals_set.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>