i386: Fix up -march=x86-64-v[234] vs. target attribute [PR98274]

i386: Fix up -march=x86-64-v[234] vs. target attribute [PR98274]

The following testcase fails to compile.  The problem is that
when ix86_option_override_internal is called the first time for command
line, it sees -mtune= wasn't present on the command line and so as fallback
sets ix86_tune_string to ix86_arch_string value ("x86-64-v2"), but
ix86_tune_specified is false, so we don't find the tuning in the table
but don't error on it.
When processing the target attribute, ix86_tune_string is what
it was earlier left with, but this time ix86_tune_specified is true and
so we error on it.

The following patch does what is done already e.g. for "x86-64" march,
in particular default the tuning to "generic".

2020-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR target/98274
	* config/i386/i386-options.c (ix86_option_override_internal): Set
	ix86_tune_string to "generic" even when it wasn't specified and
	ix86_arch_string is "x86-64-v2", "x86-64-v3" or "x86-64-v4".
	Remove useless {}s around a single statement.

	* gcc.target/i386/pr98274.c: New test.