We're getting an error when running this test on PowerPC VxWorks 7,
due to an unexpected warning:

    | Excess errors:
    | cc1: warning: '-mvsx' and '-mno-altivec' are incompatible

The warning comes from a combination of factors:
  - The test itself uses -mvsx explicitly via the following directive:
       // { dg-options "-O1 -mvsx" }
  - Our toolchain was configured so as to make -mno-altivec
    the default;
  - These two options are mutually exclusive.

This commit adds a powerpc_vsx_ok dg-require-effective-target directive
to that test, and thus making it UNSUPPORTED instead.

Tested on PowerPC VxWorks 7. Also tested on PowerPC ELF as well,
a platform where we do not make -mno-altivec the default, to verify
that the test continues to run as usual in that case.

gcc/testsuite/

        * gcc.target/powerpc/pr71763.c: Require powerpc_vsx_ok.

(cherry picked from commit c917584aa9ec32934acf0082ed383b04f49fd174)