This commit fixes __aeabi_idiv() link error when we are building
libc++abi with compiler-rt.
When compiler-rt is enabled, the option --rtlib=compiler-rt will be
added to link flags. However, if -nodefaultlibs is specified
simultaneously, --rtlib will be ignored and compiler-rt libraries will
not be linked. Consequently, several ARM built-in functions, e.g.
__aeabi_idiv(), will not be available.