[MIPS] Link .dex or .oat code lacking .MIPS.abiflags segment

This corrects an issue with mips32 Art on mips64r6 Android, where
Java ran slowly due to unintended use of kernel-trap emulation of
single-precision floating point registers. This also regressed all
Art tests due to an extra logcat line
    WARNING: linker: Using FRE=1 mode to run "..."

When targeting mips32r6, Art generates modeless or FR=1 floating point
code, same as Android's own native mips32r6 modules. So the trapping was
unneeded. Linker was confusing Art-generated modules with those from
old NDK compilers, which do need that trapping mode.

This linker filename check may become unnecessary, if Art learns how to
generate .MIPS.abiflags segments in its generated elf-like codefiles.

Change-Id: I18069d1234960c680c5df739514da09015a7fdb6

linker/linker_mips.cpp

@@ -264,10 +264,18 @@ bool soinfo::mips_check_and_adjust_fp_modes() {
 
   // FP ABI-variant compatibility checks for MIPS o32 ABI
   if (abiflags == nullptr) {
-    // Old compiles lack the new abiflags section.
-    // These compilers used -mfp32 -mdouble-float -modd-spreg defaults,
-    //   ie FP32 aka DOUBLE, using odd-numbered single-prec regs
-    mips_fpabi = MIPS_ABI_FP_DOUBLE;
+    // Old compilers and some translators don't emit the new abiflags section.
+    const char* filename = get_realpath();
+    size_t len = strlen(filename);
+    if (len > 4 && (strcmp(filename+len-4, ".dex") == 0 ||
+                    strcmp(filename+len-4, ".oat") == 0   )) {
+      // Assume dex2oat is compatible with target
+      mips_fpabi = MIPS_ABI_FP_XX;
+    } else {
+      // Old Android compilers used -mfp32 -mdouble-float -modd-spreg defaults,
+      //   ie FP32 aka DOUBLE, using FR=0 mode fpregs & odd single-prec fpregs
+      mips_fpabi = MIPS_ABI_FP_DOUBLE;
+    }
   } else {
     mips_fpabi = abiflags->fp_abi;
     if ( (abiflags->flags1 & MIPS_AFL_FLAGS1_ODDSPREG)