testsuite: require libc sym for -shared

testsuite: require libc sym for -shared

Targets whose binutils support -shared, but that don't have a shared
libc, and that can't add PDC (non-PIC) to shared libraries, may
succeed at the effective target test for -shared, because it brings
nothing from libc, but tests that rely on -shared and that use bits
from libc, such as g++.dg/lto/pr108772, fail despite requiring the
shared effective target.

Extend the effective target test to bring malloc() from libc, that's
likely to be present in libc and bring a substantial amount of code if
no shared libc is available.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_shared):
	Check for a static-only libc.