[GCC] Fix misleading aarch64 mcpu/march warning string

[Joel]

The message for conflicting mcpu and march previously printed the
architecture of the CPU instead of the CPU name, as well as omitting the
extensions to the march string. This patch corrects both errors. This
patch fixes PR target/87612.


before:
$ aarch64-unknown-linux-gnu-gcc -S -O3 -march=armv8-a+sve
-mcpu=cortex-a76 foo.c

cc1: warning: switch '-mcpu=armv8.2-a' conflicts with '-march=armv8-a'
switch

after:
$ aarch64-unknown-linux-gnu-gcc -S -O3 -march=armv8-a+sve
-mcpu=cortex-a76 foo.c

cc1: warning: switch '-mcpu=cortex-a76' conflicts with
'-march=armv8-a+sve' switch


gcc/ChangeLog:

2020-02-27  Joel Hutton  <Joel.Hutton@arm.com>
        PR target/87612
        * config/aarch64/aarch64.c (aarch64_override_options): Fix
misleading warning string.
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