[flang][test] Run Driver/fveclib-codegen.f90 for aarch64 and x86_64 (#103730)

`Flang :: Driver/fveclib-codegen.f90` currently `FAIL`s on SPARC, both
Solaris/sparcv9 and Linux/sparc64:
```
bin/flang-new -S -Ofast -fveclib=LIBMVEC -o - /vol/llvm/src/llvm-project/local/flang/test/Driver/fveclib-codegen.f90

flang/test/Driver/fveclib-codegen.f90:11:10: error: CHECK: expected string not found in input
! CHECK: _ZGVbN4vv_powf
         ^
```
The code in question only contains calls to `powf`. Given that `glibc`
only supports `libmvec` on `aarch64` and `x86_64`, this test targets
only those if possible.

Tested on `sparcv9-sun-solaris2.11`, `sparc64-unknown-linux-gnu`,
`amd64-pc-solaris2.11`, and `x86_64-pc-linux-gnu`.

Author: rorth

flang/test/Driver/fveclib-codegen.f90

@@ -1,6 +1,7 @@
 ! test that -fveclib= is passed to the backend
 ! -target aarch64 so that ArmPL is available
-! RUN: %flang -S -Ofast -fveclib=LIBMVEC -o - %s | FileCheck %s
+! RUN: %if aarch64-registered-target %{ %flang -S -Ofast -target aarch64-unknown-linux-gnu -fveclib=LIBMVEC -o - %s | FileCheck %s %}
+! RUN: %if x86-registered-target %{ %flang -S -Ofast -target x86_64-unknown-linux-gnu -fveclib=LIBMVEC -o - %s | FileCheck %s %}
 ! RUN: %flang -S -Ofast -fveclib=NoLibrary -o - %s | FileCheck %s --check-prefix=NOLIB
 
 subroutine sb(a, b)