Restore FPCR after modifying it in a test.

In particular, the test changed the rounding mode, which caused another
test to fail when run on AArch64 hardware with --run-all.
diff --git a/test/aarch64/test-assembler-aarch64.cc b/test/aarch64/test-assembler-aarch64.cc
index 5d84916..a1543a7 100644
--- a/test/aarch64/test-assembler-aarch64.cc
+++ b/test/aarch64/test-assembler-aarch64.cc
@@ -7131,6 +7131,9 @@
   __ Cinc(x7, x7, hs);  // C
   __ Cinc(x7, x7, vc);  // !V
 
+  Register old_fpcr = x15;
+  __ Mrs(old_fpcr, FPCR);
+
   // All core FPCR fields must be writable.
   __ Mov(x8, fpcr_core);
   __ Msr(FPCR, x8);
@@ -7165,6 +7168,8 @@
   __ Mrs(x10, FPCR);
 #endif
 
+  __ Msr(FPCR, old_fpcr);
+
   END();
 
   if (CAN_RUN()) {