RFR: JDK-8199052 Configure broken on aarch64

[Magnus Ihse Bursie]

 From the bug report:

It seems that JDK-8198724 "Refactor FLAGS handling in configure" broke 
the aarch64 build.

The new configure scheme tries to pass the argument "-m64" to the gcc 
compiler, but that is an x86-only flag.

configure:35228: checking whether the C compiler works
configure:35250: /usr/bin/gcc -m64 -m64 conftest.c >&5
gcc: error: unrecognized command line option '-m64'
gcc: error: unrecognized command line option '-m64'
[...]

The flags handling is currently in flux. The entire "MACHINE_FLAG" 
handling is not optimal. But we can't have aarch64 hanging around broken 
awaiting a proper refactoring, so here's a not-ideal-but-working patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199052
Patch inline:
diff --git a/make/autoconf/flags.m4 b/make/autoconf/flags.m4
--- a/make/autoconf/flags.m4
+++ b/make/autoconf/flags.m4
@@ -236,7 +236,9 @@
    if test "x$TOOLCHAIN_TYPE" = xxlc; then
      MACHINE_FLAG="-q${OPENJDK_TARGET_CPU_BITS}"
    elif test "x$TOOLCHAIN_TYPE" != xmicrosoft; then
-    MACHINE_FLAG="-m${OPENJDK_TARGET_CPU_BITS}"
+    if test "x$OPENJDK_TARGET_CPU" != xaarch64; then
+      MACHINE_FLAG="-m${OPENJDK_TARGET_CPU_BITS}"
+    fi
    fi

    # FIXME: global flags are not used yet...

/Magnus