Fix MIPS NON_MATCHING symbols when undefined symbol shadows ignored name

[tclamb]

When a base object contains an undefined symbol with the same name as a .NON_MATCHING function, object.symbol_by_name could return that undefined symbol instead of the defined function. This left the placeholder unignored and erroneously reported as matched.

Generated test ELF using mwccpsp & mwccgap from the following sources:

foo.cpp:

#include "common.h"
extern "C" void foo();
extern "C" void bar(){ foo(); }
INCLUDE_ASM("foo", foo);

foo.s:

.include "macro.inc"
.set noat
.set noreorder
nonmatching foo, 0x8
glabel foo
    nop
    nop
endlabel foo

Added test fails without patch.

Verified that my repo's report.json no longer over-reports these as matches.